[InstCombine] Use commutable and/or/xor matchers to simplify some code

Summary:
This is my first time using the commutable matchers so wanted to make sure I was doing it right.

Are there any other matcher tricks to further shrink this? Can we commute the whole match so we don't have to LHS and RHS separately?

Reviewers: davide, spatel

Reviewed By: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31680

llvm-svn: 299840