[x86] Refactor some of the new code for lowering v16i8 shuffles to
remove duplication and make it easier to select different strategies.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212674 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed