[X86][SSE] Lower shuffles to permute(unpack(x,y)) (PR31151)

Attempt to lower a shuffle as an unpack of elements from two inputs followed by a single-input (wider) permutation.

As long as the permutation is wider this is a win - there may be some circumstances where same size permutations would also be useful but I've left that for future work.

Differential Revision: https://reviews.llvm.org/D52043

llvm-svn: 342257