[InstCombine] fold signbit test of a pow2-or-zero

[rotateright]

This probably isn't worth the compile-time cost of using ValueTracking, but we could add a direct pattern match for this and similar icmp patterns:

define i1 @src(i8 %x) {
  %negx = sub i8 0, %x
  %pow2_or_zero = and i8 %negx, %x 
  %cmp = icmp slt i8 %pow2_or_zero, 0
  ret i1 %cmp
}

define i1 @tgt(i8 %x) {
  %cmp = icmp eq i8 %x, -128
  ret i1 %cmp
}

https://alive2.llvm.org/ce/z/_J5q3S

[junaire]

I would like to have a try :)

[junaire]

Candidate patch: https://reviews.llvm.org/D144777