[X86][SSE] Lower constant vXi8 ISD::SRL/ISD::SRA using PMULLW

Extending the concept introduced in D49562, this patch lowers constant vXi8 ISD::SRL/ISD::SRA by zero/sign extending to vXi16 and using PMULLW and then truncating the high 8 bits of the result.

Differential Revision: https://reviews.llvm.org/D50781

llvm-svn: 340062