[X86][SSE] Add v16i8/v32i8 multiplication support

Patch to allow int8 vectors to be multiplied on the SSE unit instead of being scalarized.

The patch sign extends the i8 lanes to i16, uses the SSE2 pmullw multiplication instruction, then packs the lower byte from each result.

Differential Revision: http://reviews.llvm.org/D9115

llvm-svn: 235837