Update HInstruction::NeedsCurrentMethod.

HLoadString and HLoadClass when sharpened may not need it
anymore. Instead just rely on the HCurrentMethod being the
SSA dependency of those instructions.

Also save storing the current method in the stack if the
graph actually doesn't need it.

test: m test-art-host test-art-target
Change-Id: I235d8275230637cbbd38fc0d2f9b822f6d2a9c1e