RFR 8202201: All oop stores in the x64 interpreter are treated as volatile when using G1

[coleen.phillimore at oracle.com]

Summary: ran out of registers, generated volatile and non-volatile branches.

open webrev at http://cr.openjdk.java.net/~coleenp/8202201.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8202201

Tested with applications/jcstress, tier1&2 and jvmti tests (sometimes 
they filled up interpreter fixed space).

putfield bytecode size is 2176 bytes now, 1248 bytes before
putstatic bytecode size is 1344 bytes now, 864 before
fast_xputfield is same 96 bytes.

Thanks,
Coleen