Add intrinsic for Reference.get()

Added an intrinsic function for Reference.get(). Return immediately
without going through JNI if the slow path is not currently in use.
Otherwise, branch off to the the existing JNI function.

Approximately 47x speedup for cases where slow path is not enabled.

Change-Id: I13ad65a356fe4e104d8d83980694dc2740d7d039
14 files changed