[InstCombine] cttz(sext(x)) -> cttz(zext(x))

```

----------------------------------------
define i32 @src(i16 %x, i1 %b) {
%0:
  %z = sext i16 %x to i32
  %p = cttz i32 %z, %b
  ret i32 %p
}
=>
define i32 @tgt(i16 %x, i1 %b) {
%0:
  %z = zext i16 %x to i32
  %p = cttz i32 %z, %b
  ret i32 %p
}
Transformation seems to be correct!
```

https://alive2.llvm.org/ce/z/evomeg

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D101764