[PowerPC] [NFC] Cleanup xxpermdi peephole optimization
Following on from rG884351547da2, this patch cleans up the logic for xxpermdi
peephole optimizations by converting two layers of nested ifs to early breaks
and simplifying the logic.
Reviewers: hfinkel, nemanjai, jsji, lkail, #powerpc, steven.zhang
Reviewed By: #powerpc, steven.zhang
Subscribers: wuzish, steven.zhang, hiraditya, kbarton, shchenz, llvm-commits
Differential Revision: https://reviews.llvm.org/D71170
Patch by vddvss (Colin Samples).