[x86] More refactoring of the shuffle comment emission. The previous

[x86] More refactoring of the shuffle comment emission. The previous
attempt didn't work out so well. It looks like it will be much better
for introducing extra logic to find a shuffle mask if the finding logic
is totally separate. This also makes it easy to sink the opcode logic
completely out of the routine so we don't re-dispatch across it.

Still no functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218363 91177308-0d34-0410-b5e6-96231b3b80d8