[PowerPC] [NFC] Cleanup xxpermdi peephole optimization

Summary:
Following on from rG884351547da2, this patch cleans up the logic for `xxpermdi`
peephole optimizations by converting two layers of nested `if`s 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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71170

Patch by vddvss (Colin Samples).