InstSimplify: Add a hook for shufflevector

Summary:
Add a hook for simplification of shufflevector's with the following rules:
- Constant folding - NFC, as it was already being done by the default handler.
-  If only one of the operands is constant, constant fold the shuffle if the
    mask does not select elements from the variable operand -  to show the hook is firing and affecting the test-cases.

Reviewers: RKSimon, craig.topper, spatel, sanjoy, nlopes, majnemer

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31525

llvm-svn: 299393