arm/arm64: Relax register allocation for Baker RB. The Baker RB implementation with introspection does not need the holder register anymore when it loads the reference, so we can allow the output to be in the same register. Test: testrunner.py --optimizing --target Bug: 344649052 Change-Id: I7ef745abcbd7236f12d0ac1bee6e73427e95c8a2