[x86] Teach the new vector shuffle lowering a fancier way to lower

[x86] Teach the new vector shuffle lowering a fancier way to lower
256-bit vectors with lane-crossing.

Rather than immediately decomposing to 128-bit vectors, try flipping the
256-bit vector lanes, shuffling them and blending them together. This
reduces our worst case shuffle by a pretty significant margin across the
board.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218446 91177308-0d34-0410-b5e6-96231b3b80d8