[x86] Eliminate `movzx` when upper part of register is irrelevant or overwritten

```zig
export fn foo(x: u8) u32 {
return @ctz(x >> 1);
}
```

```asm
foo:
shr dil
movzx eax, dil
or eax, 256
tzcnt eax, eax
ret
```

Could be:

```asm
foo:
shr dil
or edi, 256
tzcnt eax, edi
ret
```

The compiler could also look for things like `or eax, -256` and eliminate `movzx eax, al` since the relevant bits are overwritten anyway.