InstCombine: Use the InstSimplify hook for shufflevector

Summary: Start using the recently added InstSimplify hook for shuffles in the respective InstCombine visitor.

Reviewers: spatel, RKSimon, craig.topper, majnemer

Reviewed By: majnemer

Subscribers: majnemer, llvm-commits

Differential Revision: https://reviews.llvm.org/D31526

llvm-svn: 299412