[x86] Teach the unpack lowering to try wider element unpacks.

[x86] Teach the unpack lowering to try wider element unpacks.

This allows it to match still more places where previously we would have
to fall back on floating point shuffles or other more complex lowering
strategies.

I'm hoping to replace some of the hand-rolled unpack matching with this
routine is it gets more and more clever.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229463 91177308-0d34-0410-b5e6-96231b3b80d8