[RS4GC] Handle ShuffleVector instruction in findBasePointer

Summary:
This patch modifies the findBasePointer to handle the shufflevector instruction.

Tests run: RS4GC tests, local downstream tests.

Reviewers: reames, sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25197

llvm-svn: 283219