X86: Update isVectorShiftByScalarCheap with cases covered by AVX512BW

Summary:
AVX512BW adds support for variable shift amount for 16-bit element
vectors.

Reviewers: craig.topper, RKSimon, spatel

Reviewed By: RKSimon

Subscribers: rengolin, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D42437

llvm-svn: 323292