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

Change-Id: Iea6a7f430009f37a9599b0a0039207049906e45d
13 files changed