RFR: 8227083: ZGC: C2: Generates on_weak barrier for WeakCompareAndSwap

[Stefan Karlsson]

Hi all,

Please review this patch to fix so that we generate an on_strong load 
barrier for WeakCompareAndSwap.

http://cr.openjdk.java.net/~stefank/8227083/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8227083

ErikÖ spotted this problem when we were looking into the bug for 
JDK-8225770, where C2 also generated the wrong kind of barrier.

In this bug there was a mix-up of terms, the weak nature of 
WeakCompareAndSwap is unrelated to the on_weak property of some load 
barriers. The barrier in this code path should be using the normal, 
on_strong, load barries.

The verification code in JDK-8227085 did catch this bug, as expected, 
and with the fix for this bug we don't fail in the verification code 
anymore.

Tested with tier 1-7.

Thanks,
StefanK