[x86] Begin teaching the new vector shuffle lowering among the most
important bits of cleverness: to detect and lower repeated shuffle
patterns between the two 128-bit lanes with a single instruction.

This patch just teaches it how to lower single-input shuffles that fit
this model using VPERMILPS. =] There is more that needs to happen here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218211 91177308-0d34-0410-b5e6-96231b3b80d8