RFR: 8077364 "if( !this )" construct prevents build on Xcode 6.3

[Staffan Larsen]

Please review this fix for compiling hotspot with clang 6.1.

I have verified that the compiler workarounds are still needed for these tests:
jdk/test/sun/misc/CopyMemory.java
hotspot/test/compiler/c2/5091921/

webrev: http://cr.openjdk.java.net/~sla/8077364/webrev.00 <http://cr.openjdk.java.net/~sla/8077364/webrev.00>
bug: https://bugs.openjdk.java.net/browse/JDK-8077364 <https://bugs.openjdk.java.net/browse/JDK-8077364>

Thanks,
/Staffan