[InstCombine] fold bit-hack form of signbit test

[rotateright]

Missed fold in IR noticed while trying to fix #57381:

define i32 @src(i16 %x) {
  %a = ashr i16 %x, 15 ; -1 if negative, 0 if not-negative
  %r = add i16 %a, 1   ; 0 if negative, 1 if not-negative
  %r2 = zext i16 %r to i32
  ret i32 %r2
}

define i32 @tgt(i16 %x) {
  %a = icmp sgt i16 %x, -1
  %r2 = zext i1 %a to i32
  ret i32 %r2
}

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

[rotateright]

We miss the more basic fold without the cast:
https://alive2.llvm.org/ce/z/5CrWQR