[RS4GC] Look through vector bitcasts when looking for base pointer

[RS4GC] Look through vector bitcasts when looking for base pointer

Summary:
 In RS4GC it is possible that a base pointer is contained in a vector that
has undergone a bitcast from one element-pointertype to another. We teach
RS4GC how to look through bitcasts of vector types when looking for a base
pointer.

Reviewers: anna

Reviewed By: anna

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38849

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315694 91177308-0d34-0410-b5e6-96231b3b80d8