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