zero broken by removal of implicit oop to Handle conversions

JDK-8169881 removed the implicit conversion from oop -> Handle. A consequence of this is that Handle::operator=(const Handle&) is no longer applicable to an oop argument. There is code in bytecodeInterpreter.cpp (only used by zero) that attempts to assign an oop to a handle, and fails to compile because of that change.

There might be other zero-specific places with a similar problem; bytecodeInterpreter.cpp is just the first one I encountered when attempting to do a zero build.