RFR(s): 8013171: G1: C1 x86_64 barriers use 32-bit accesses to 64-bit PtrQueue::_index

[Per Liden]

Hi,

(This change affects G1, but it's touching code in C1 so I'd like to ask 
someone from the compiler team to also reviewed this)

Summary: The G1 barriers loads and updates the PrtQueue::_index field. 
This field is a size_t but the C1 version of these barriers aren't 
64-bit clean. The bug has more details.

In addition I've massaged the code a little bit, so that the 32-bit and 
64-bit sections look more similar (and as a bonus I think we avoid an 
extra memory load on 32-bit).

Webrev: http://cr.openjdk.java.net/~pliden/8013171/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8013171

Testing:
* gc-test-suite on both 32 and 64-bit builds (with -XX:+UseG1GC 
-XX:+TieredCompilation -XX:TieredStopAtLevel=3 -XX:+VerifyAfterGC)
* Passes jprt

/Per