[InstCombine] add tests for icmp gt/lt (shr X, C1), C2; NFC

Surprisingly, we have zero coverage for these patterns.

Many of these are handled in InstSimplify, but it's not obvious
what the rule for folding each case should be, so I've just
stamped out everything.

It should be possible to fold every case, but currently, we
miss these:

int ashr_slt(int x) {
  return (x >> 1) < 1; 
}

int ashr_sgt(int x) {
  return (x >> 1) > 0; 
}

https://godbolt.org/g/aB2hLE

llvm-svn: 314837