[RS4GC] Fix crash in the case that a live variable has a constant base.

Summary:
Previously, RS4GC crashed in CreateGCRelocates() because it assumed
that every base is also in the array of live variables, which isn't true if a
live variable has a constant base.

This change fixes the crash by making sure CreateGCRelocates() won't try to
relocate a live variable with a constant base.  This would be unnecessary
anyway because anything with a constant base won't move.

Reviewers: reames

Subscribers: llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D15556

llvm-svn: 256252