review (S) for 6992477: fix for 6991512 broke sparc barriers

[Tom Rodriguez]

http://cr.openjdk.java.net/~never/6992477

6992477: fix for 6991512 broke sparc barriers
Reviewed-by:

The fix for 6991512 changed the type of the temporary register used
for the address of the field being modified in a Object cmpxchg from
intptr_t to T_OBJECT.  T_OBJECT is only used for pointers to the head
of an object and the result of this calculation is a derived pointer
which causes later failures when trying to emit code.  The fix is to
restore the original code and then fix the original issue by changing
the G1PostBarrierStub::emit_code on x86 to use as_pointer_register to
match the sparc version of the code.  Tested with failing tests from
nightly.

src/cpu/sparc/vm/c1_LIRGenerator_sparc.cpp
src/cpu/x86/vm/c1_LIRGenerator_x86.cpp
src/share/vm/c1/c1_LIRGenerator.cpp
src/cpu/x86/vm/c1_CodeStubs_x86.cpp