[InstCombine] fold abs(X)/X to cmp+select

The backend can convert the select-of-constants to
bit-hack shift+logic if desirable.

https://alive2.llvm.org/ce/z/pgJT6E

  define i8 @src(i8 %x) {
  %0:
    %a = abs i8 %x, 1
    %d = sdiv i8 %x, %a
    ret i8 %d
  }
  =>
  define i8 @tgt(i8 %x) {
  %0:
    %cond = icmp sgt i8 %x, 255
    %r = select i1 %cond, i8 1, i8 255
    ret i8 %r
  }
  Transformation seems to be correct!