[InstCombine] try harder to form select from logic ops

This is part of solving PR37549:
https://bugs.llvm.org/show_bug.cgi?id=37549

The patterns shown here are a special case of something
that we already convert to select. Using ComputeNumSignBits()
catches that case (but not the more complicated motivating
patterns yet).

The backend has hooks/logic to convert back to logic ops
if that's better for the target.

llvm-svn: 344609