RFR (S) 8227766: CheckUnhandledOops is broken in MemAllocator

[coleen.phillimore at oracle.com]

Summary: Save oop created in handle more eagerly, so CheckUnhandledOops 
doesn't bash it.

Also added a test for the case that was failing.  There were a couple of 
changes needed to compile CHECK_UNHANDLED_OOPS with slowdebug, but I 
reverted the makefile changes for doing that because I don't think we 
want it for slowdebug.
Lastly, made some changes to make CheckUnhandledOops easier to debug, 
and removed an unnecessary clearing, since it's also done in the transition.

Tested with hs-tier1-3 with -XX:+CheckUnhandledOops and without.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8227766.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8227766

Thanks,
Coleen