Footguns

Trivial plain C counterexample, where the check is seen by the compiler, but the programmer did not intend to tell the compiler that the pointer could be NULL:

/* In a header file, for config handling */
struct configs {
/* Lots of pointers to config members here, omitted */
};

#define GET_STR_CONFIG_OR_DEFAULT(config, member, default) \
    (config ? config->member ? config->member : default : default)

#define GET_INT_CONFIG_OR_DEFAULT(config, member, default)
    (config ? config->member ; default)

/* In a C file, using the header file */
void do_amazing_things(void *config) {
    int rating = GET_INT_CONFIG_OR_DEFAULT(config, rating, 0);
    char * expertise = GET_STR_CONFIG_OR_DEFAULT(config, expertise, "novice");

    /* Do the amazing things */
}

In this case, the call to GET_INT_CONFIG_OR_DEFAULT (a macro) has promised that config is not NULL because it does an early return; a compiler that doesn't optimize this will be issuing repeated NULL checks for something that it knows must be non-NULL, or complaining because the user has an unnecessary NULL check in the expansion of GET_STR_CONFIG_OR_DEFAULT.

The code the compiler sees when the preprocessor hands it off is:

struct configs {
/* Lots of pointers to config members here, omitted */
};
/* Rest of headers etc */

void do_amazing_things(void *config) {
    int rating = (config ? config->member : 0);
    char * expertise = (config ? config->member ? "expertise" : "expertise");
}

Which leads you into the question of provenance of your "always true" if statement; in this case, the compiler has to know that it knows config is not NULL because the programmer did something that's an explicit check, not a "if it's NULL, then we have UB" case, otherwise you get a noisy warning because the programmer included an unnecessary NULL check in the macro. But they did that because that way the macro is safe to use even if it's only used once in a function, and if the user does do:

/* In a C file, using the header file */
void do_amazing_things(void *config) {
    int rating = ((struct configs*) config)-gt;rating;
    char * expertise = GET_STR_CONFIG_OR_DEFAULT(config, expertise, "novice");

    /* Do the amazing things */
}

then the compiler will need to warn because its "config is not NULL" assumption came from "if config is NULL, then there is UB in the execution". But first we need compiler authors to get onboard with the idea that whenever UB is invoked to permit an optimization (noting that this can require you to do complex tracking to get from your low-level IR (MachineInstr in the LLVM case, for example) all the way back to C, so that you can confirm that a given decision in codegen is not causing issues because it results in bad signed overflow behaviour (for example).

But the compiler may not know that. E.g. you could have code like:

struct S {
    int n;
    // ...
};

template <typename T, typename Pred>
void reset_if(T &items, Pred pred) {
    for (auto &item : items)
        if (pred(item))
            item = nullptr;
}

// Precondition: all items are not nullptr
// Postcondition: all items are even-numbered or are nullptr
void remove_odd(std::vector<std::unique_ptr<S>> &items)
{
    reset_if(items, [](auto &item) { return item->n % 2 != 0; });
}

(i.e. there's a dynamically-allocated array of pointers to S, and the function deallocates all the Ss that match some condition. This is somewhat artificial, but I don't think it's totally implausible as a component of some larger operation). The function's precondition is guaranteed by the programmer but is not visible to the compiler. (Maybe the caller is in a separate translation unit, or a dynamically-loaded library.)

On "item = nullptr", the unique_ptr will deallocate its original contents (to prevent memory leaks). Because unique_ptr is generic and supports custom deallocator functions, and the custom deallocator might not accept nullptr as an argument, the unique_ptr destructor is specified to do something like "if (get() != nullptr) get_deleter()(get());".

In this function, the unique_ptr destructor is only called after the "item->n % 2" check, which is dereferencing the pointer. Then the compiler knows the pointer can't be nullptr, so it can omit the "if (get() != nullptr)" test and save some time and code size. (In this case it's only a couple of instructions, but it's not hard to imagine similar cases where the 'if' is followed by an 'else' with a much larger chunk of code, and eliminating the whole 'else' clause could make a big difference.)

The null check wasn't intentionally added to this code by any programmer, it's just a part of the (relatively) safe abstraction provided by unique_ptr, so it's included in the code that's generated by the template instantiation. In this function the code is used in a way where the check is redundant and the compiler is able to figure that out because it sees the programmer was deliberately dereferencing the pointer, so the abstraction has zero cost and the programmer doesn't have to revert to a less-safe kind of pointer to get the same performance.

And the problem with your point is that C is not very amenable to whole program analysis (for a variety of reasons to do with only having a global and a local static namespace, plus using textual inclusion instead of modules), which means that there's a lot of legacy code where the first dereference is not UB. It's only UB if the program execution can call ub with forty being NULL, which is not guaranteed. If the rest of the code ensures that ub is always called with a valid pointer to configs, then no UB is present.

As I've pointed out in another comment, that check could be the consequence of a macro expansion - in which case, pessimising the output machine code because you've used a macro to access configs isn't a good look - there was no UB before, now you're adding code that verifies something that the human writing the code has already verified is true outside ub, and the use of a macro has been sufficient to introduce extra machine instructions.

This is part of why it's a damn hard social problem - it's possible that ub is only called from code in 3 different C files, and all of those files already check for NULL before calling ub, so in the actual final application, there is no UB. The compiler using the fact that the check can only be true and cause an early return if UB has already happened to optimize ub simply speeds things up at no cost.