[InstCombine] fold icmp equality with 'or' mask ops

This could go either direction since the instruction
count is the same either way, but there are a few
reasons to prefer this:
1. We already do the related transform with 'and'
   (see just above the new code).
2. We try (too hard) to compensate for not having this
   and possibly other folds in transformZExtICmp(),
   and that leads to bugs like https://llvm.org/PR51762 .
3. Codegen looks better across a variety of targets.

https://alive2.llvm.org/ce/z/uEgn4P