IR: Add a shufflevector mask commutation helper function. NFC.

Summary:
Following up on Sanjay's suggetion in D32955, move this functionality
into ShuffleVectornstruction.

Reviewers: spatel, RKSimon

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32956

llvm-svn: 302420