[InstCombine] rotate(X,Z) eq/ne rotate(Y,Z) ---> X eq/ne Y (PR51565)

[InstCombine] rotate(X,Z) eq/ne rotate(Y,Z) ---> X eq/ne Y (PR51565)

```

----------------------------------------
define i1 @src(i8 %x, i8 %y, i8 %z) {
%0:
  %f = fshl i8 %x, i8 %x, i8 %z
  %f2 = fshl i8 %y, i8 %y, i8 %z
  %r = icmp eq i8 %f, %f2
  ret i1 %r
}
=>
define i1 @tgt(i8 %x, i8 %y, i8 %z) {
%0:
  %r = icmp eq i8 %x, %y
  ret i1 %r
}
Transformation seems to be correct!

```

https://alive2.llvm.org/ce/z/qAZp8f

Solves PR51565

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D109271