Why can’t the C compiler optimize changing the value of a const pointer assuming that two pointers to the same variable would be illegal/UB?

Why can’t the C Compiler optimize the first line away, under the assumption, that two pointers to the same variable would be illegal/UB?

Because you haven’t instructed the C compiler to do so — that it is allowed to make that assumption.

C has a type qualifier for exactly this called restrict which roughly means: this pointer does not overlap with other pointers (not exactly, but play along).

The assembly output for

bool f(int* restrict a, const int* b) {
  *a = 2;
  int ret = *b;
  *a = 3;
  return ret != 0;
}

is

        mov     eax, DWORD PTR [rsi]
        mov     DWORD PTR [rdi], 3
        test    eax, eax
        setne   al
        ret

… which removes/optimizes-away the assignment *a = 2

From https://en.wikipedia.org/wiki/Restrict

In the C programming language, restrict is a keyword that can be used in pointer declarations. By adding this type qualifier, a programmer hints to the compiler that for the lifetime of the pointer, only the pointer itself or a value directly derived from it (such as pointer + 1) will be used to access the object to which it points.