[X86][SSE] Support v8i16/v16i16 rotations

Extension to D46954 (PR37426), this patch adds support for v8i16/v16i16 rotations in a similar manner - the conversion of the shift/rotate amount to a multiplication factor and the use of PMULLW to shift left and PMULHUW (ISD::MULHU) to shift the wrapped bits back around to be ORd together.

Differential Revision: https://reviews.llvm.org/D47822

llvm-svn: 334309