[x86] Hoist the actual lowering logic into a helper function to separate

[x86] Hoist the actual lowering logic into a helper function to separate
it from the shuffle pattern matching logic.

Also cleaned up variable names, comments, etc. No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218152 91177308-0d34-0410-b5e6-96231b3b80d8