[X86][SSE] Match zero/any extension shuffles that don't start from the first element

This patch generalizes the lowering of shuffles as zero extensions to allow extensions that don't start from the first element. It now recognises extensions starting anywhere in the lower 128-bits or at the start of any higher 128-bit lane.

The motivation was to reduce the number of high cost pshufb calls, but it also improves the SSE2 case as well.

Differential Revision: http://reviews.llvm.org/D12561

llvm-svn: 248250