Add a GVN dependency 'GC' for garbage collection.

This will be used by incoming architecture specific optimizations. The
dependencies must be conservative. When an HInstruction is created we
may not be sure whether it can trigger GC. In that case the
'ChangesGC' dependency must be set. We control at code-generation time
that HInstructions that can call have the 'ChangesGC' dependency
set.

Change-Id: Iea6a7f430009f37a9599b0a0039207049906e45d