Vector forms of SHL, SRA, and SRL can be constant folded using SimplifyVBinOp too

Reviewers: dsanders

Reviewed By: dsanders

CC: llvm-commits, nadav

Differential Revision: http://llvm-reviews.chandlerc.com/D1958

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194393 91177308-0d34-0410-b5e6-96231b3b80d8