RFR (S): 8250628: ZGC: `fixup_partial_loads` was removed, but still are referenced

[Albert Yang]

Hi,

https://bugs.openjdk.java.net/browse/JDK-8250628

http://cr.openjdk.java.net/~lkorinth/albert/8250628/0/

Case 2&3 can be removed, since all roots will go through the load 
barrier before a safepoint. To be safe, I have tried setting them to 
true via `FLAG_SET_DEFAULT(_, true);`, and no failures are observed for 
hotspot-gc.

Tested: hotspot-gc

/Albert