[InstCombine] Masked-merge fold `(x^y)&~m ^ y` -> `(x^y)&m ^ x` introduces more undef

InstCombine canonicalizes the masked-merge

```
y ^ ((x ^ y) & ~m)
```

into

```
x ^ ((x ^ y) & m)
```

his is only valid if the two mask polarities are interchangeable, which is correct for most cases. But it might be incorrect when `%x = undef` in following transform:

Source:
```llvm
define i4 @scalar(i4 %x, i4 %y, i4 %m) {
%im = xor i4 %m, -1
%n0 = xor i4 %x, %y
%n1 = and i4 %n0, %im
%r = xor i4 %n1, %y
ret i4 %r
}
```

Target:
```llvm
define i4 @scalar(i4 %x, i4 %y, i4 %m) local_unnamed_addr #0 {
%n0 = xor i4 %y, %x
%1 = and i4 %n0, %m
%r = xor i4 %1, %x
ret i4 %r
}
```

Reproduce: https://alive2.llvm.org/ce/z/XEiEYr