[mips] Analyze and provide selection patterns microMIPSR6 branches

These branches were previously unanalyzable and unselectable. Add them and
recognize how to generate their inverses.

Reviewers: smaksimovic, atanasyan, abeserminji

Differential Revision: https://reviews.llvm.org/D46113


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331050 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed