[RS4GC] Fix rematerialization of bitcast of bitcast.

Summary:
Previously, only the outer (last) bitcast was rematerialized, resulting in a
use of the unrelocated inner (first) bitcast after the statepoint.  See the
test case for an example.

Reviewers: igor-laevsky, reames

Subscribers: reames, alex, llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D15789

llvm-svn: 256520