[Bug middle-end/127285] GCC generates redundant `test + sete` with memcmp against 0 with a constant size

[saicharan at astrome dot co via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127285

--- Comment #2 from Sai Charan Marrivada Gopala Krishna <saicharan at astrome 
dot co> ---
```
"test(char*, char*)":
        movzx   edx, WORD PTR [rdi]
        xor     eax, eax
        cmp     WORD PTR [rsi], dx
        je      .L5
        ret
.L5:
        movzx   eax, BYTE PTR [rsi+2]
        cmp     BYTE PTR [rdi+2], al
        sete    al
        ret
```
is even better. GCC16.2 generates the above assembly for the following code.

```
bool test(char *a, char *b) {
    if (__builtin_memcmp(a, b, 2) == 0) {
        return a[2] == b[2];
    }
    return false;
}
```
Compiler explorer link: https://godbolt.org/z/jfxdEKnE8