RFR(XXS): 7036706 G1: Use LIR_OprDesc::as_pointer_register in code changes for 7035117

[John Cuthbertson]

Hi Everyone,

  Can I have a couple of volunteers to review this change? The webrev 
can be found at: http://cr.openjdk.java.net/~johnc/7036706/webrev.0/

The change incorporates Christian's suggestion to use 
LIR_OprDesc::as_pointer_register() to define thread_reg in the C1 code 
stubs for Unsafe.getObject.

Testing: nsk stress tests (including jni stress tests) with 
TieredCompilation on both sparc64 and x64.

Thanks

JohnC