[InstCombine] clean up matchDeMorgansLaws(); NFCI

[InstCombine] clean up matchDeMorgansLaws(); NFCI

The motivation for getting rid of dyn_castNotVal is to allow fixing:
https://bugs.llvm.org/show_bug.cgi?id=32706

So this was supposed to be functional-change-intended for the case
of inverting constants and applying DeMorgan. However, I can't find
any cases where that pattern will actually get to matchDeMorgansLaws()
because we have other folds in visitAnd/visitOr that do the same
thing. So this ends up just being a clean-up patch with slight efficiency
improvement, but no-functional-change-intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302581 91177308-0d34-0410-b5e6-96231b3b80d8