RFR(S) 8190817: deopt special-case for _return_register_finalizer is confusing and leads to bugs

https://bugs.openjdk.java.net/browse/JDK-8190817 http://cr.openjdk.java.net/~dlong/8190817/webrev/ This fix replaces the problematic use of _normal_table.entry(Bytecodes::_return).entry(vtos) as a deoptimization entry point with a proper deopt entry point returned by deopt_reexecute_return_entry().  This is needed to handle the situation where compiled code is calling register_finalizer() and gets deoptimized. I also noticed that we generate duplicate entry points unnecessarily, so I cleaned that up at the same time. aarch64/ppc64/s390 folks, please check that compiler/runtime/Test8168712.java still passes. dl