[x86] Restructure the checking patterns for v16 and v32 avx2 vector
shuffle lowering to allow much better blend matching.

Specifically, with the new structure the code seems clearer to me and we
correctly can hit the cases where merging two 128-bit lanes is a clear
win and can be shuffled cheaply afterward.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222539 91177308-0d34-0410-b5e6-96231b3b80d8