[X86][SSE] Use ISD::MULHU for constant/non-zero ISD::SRL lowering (PR38151)

As was done for vector rotations, we can efficiently use ISD::MULHU for vXi8/vXi16 ISD::SRL lowering.

Shift-by-zero cases are still problematic (mainly on v32i8 due to extra AND/ANDN/OR or VPBLENDVB blend masks but v8i16/v16i16 aren't great either if PBLENDW fails) so I've limited this first patch to known non-zero cases if we can't easily use PBLENDW.

Differential Revision: https://reviews.llvm.org/D49562

llvm-svn: 338407