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

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

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

Could be:

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.