GlobalOptPass turns global that only takes two values into a bool even if the values can be represented by a byte on the target

The following program:

```c
static int v;

void maybe_called_from_another_tu() {
v = 123;
}

int main() {
return v;
}
```

gets compiled into:

```asm
maybe_called_from_another_tu:
mov byte ptr [rip + v], 1
ret

main:
xor ecx, ecx
cmp byte ptr [rip + v], 0
mov eax, 123
cmove eax, ecx
ret
```

even though it could've been:

```asm
maybe_called_from_another_tu:
mov byte ptr [rip + v], 123
ret

main:
movzx eax, byte ptr [rip + v]
ret
```

This optimisation would and does make more sense if the value written was greater than 255 but the i32 turning into an i1 doesn't make sense to me when it ends up being stored as an i8 that could've stored the actual value in this case.

godbolt: https://godbolt.org/z/EsoabcGWP