[RS4GC] Pass DenseMap by reference, NFC

Summary:
Passing the rematerialized values map to insertRematerializationStores by
value looks to be a simple oversight; update it to pass by reference.


Reviewers: reames, sanjoy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16911

llvm-svn: 259867