[x86] Refactor the bit shift code the same as I just did the byte shift

code.

While this didn't have the miscompile (it used MatchLeft consistently)
it missed some cases where it could use right shifts. I've added a test
case Craig Topper came up with to exercise the right shift matching.

This code is really identical between the two. I'm going to merge them
next so that we don't keep two copies of all of this logic.

llvm-svn: 229655