[PatternMatch] allow intrinsic form of min/max with existing matchers

I skimmed the existing users of these matchers and don't see any problems
(eg, the caller assumes the matched value was a select instruction without checking).

So I think we can generalize the matching to allow the new intrinsics or the cmp+select idioms.

I did not find any unit tests for the matchers, so added some basics there. The instsimplify
tests are adapted from existing tests for the cmp+select pattern and cover the folds in
simplifyICmpWithMinMax().

Differential Revision: https://reviews.llvm.org/D85230