[X86][SSE] Fold scalar horizontal add/sub for non-0/1 element extractions

We already perform horizontal add/sub if we extract from elements 0 and 1, this patch extends it to non-0/1 element extraction indices (as long as they are from the lowest 128-bit vector).

Differential Revision: https://reviews.llvm.org/D61263

llvm-svn: 359707