[x86] Start improving the matching of unpck instructions based on test
cases from Halide folks. This initial step was extracted from
a prototype change by Clay Wood to try and address regressions found
with Halide and the new vector shuffle lowering.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221779 91177308-0d34-0410-b5e6-96231b3b80d8