RFR (S) 8214972: Uses of klass_holder() except GC need to apply GC barriers

[coleen.phillimore at oracle.com]

Summary: Fix klass_holder() and make all callers use it, remove 
holder_phantom().*

*This removes the confusion about which one to use, except for GC, who 
has to know.

open webrev at http://cr.openjdk.java.net/~coleenp/8214972.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8214972

Ran tier1-3 tests.

Thanks,
Coleen
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