[InstCombine] reduce signbit test of logic ops to cmp with zero

This is the pattern from the description of:
https://llvm.org/PR50816

There might be a way to generalize this to a smaller or more
generic pattern, but I have not found it yet.

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

define i1 @src(i8 %x) {
  %add = add i8 %x, -1
  %xor = xor i8 %x, -1
  %and = and i8 %add, %xor
  %r = icmp slt i8 %and, 0
  ret i1 %r
}

define i1 @tgt(i8 %x) {
  %r = icmp eq i8 %x, 0
  ret i1 %r
}