[IndVars] Strengthen inference of samesign flags (#170363)
When reviewing another change, I noticed that we were failing to infer
samsign for two cases: 1) an unsigned comparison, and 2) when both
arguments were known negative.
Using CVP and InstCombine as a reference, we need to be careful to not
allow eq/ne comparisons. I'm a bit unclear on the why of that, and for
now am going with the low risk change. I may return to investigate that
in a follow up.
Compile time results look like noise to me, see:
https://llvm-compile-time-tracker.com/compare.php?from=49a978712893fcf9e5f40ac488315d029cf15d3d&to=2ddb263604fd7d538e09dc1f805ebc30eb3ffab0&stat=instructions:u
c752bb9203