BBVectorize: Check the input types of shuffles for legality

This fixes a bug where shuffles were being fused such that the
resulting input types were not legal on the target. This would
occur only when both inputs and dependencies were also foldable
operations (such as other shuffles) and there were other connected
pairs in the same block.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167731 91177308-0d34-0410-b5e6-96231b3b80d8