InstructionSimplify: One getShuffleMask() replacing multiple getMaskValue(). NFC.

Summary: This is a preparatory step for D32338.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon, spatel

Subscribers: spatel, llvm-commits

Differential Revision: https://reviews.llvm.org/D32388

llvm-svn: 301765