X86: Optimize GenCAS

This commit adds a small optimization to GenCAS: skip the MarkGCCard if
the value was not set (cmpxchg fails).

Test: ART_HEAP_POISONING=true art/test.py --host --all-compiler -r --32
Test: ART_HEAP_POISONING=false art/test.py --host --all-compiler -r --32
Test: ART_USE_READ_BARRIER=true art/test.py --host --all-compiler -r --32
Test: ART_USE_READ_BARRIER=false art/test.py --host --all-compiler -r --32
Change-Id: Ic483c4ccb0a58f7c21e6fd9f3bc90ba8c56492b8
1 file changed