Cleanup: Remove unnecessary use of ReturnValueRegister

Change-Id: Ic159fa8dcbce3d51db0b39d1cb22d01f9a94e15a
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
JIT: support WIN64

Change-Id: Id657831231ddec66f7d41f32aaac22a71c4c1673
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>