[SPARC] Use AtomicExpandPass to expand AtomicRMW instructions.

They were previously expanded to CAS loops in a custom isel expansion,
but AtomicExpandPass knows how to do that generically.

Testing is covered by the existing sparc atomics.ll testcases.

llvm-svn: 264771
diff --git a/llvm/lib/Target/Sparc/SparcISelLowering.h b/llvm/lib/Target/Sparc/SparcISelLowering.h
index c470fa8..5ee0256 100644
--- a/llvm/lib/Target/Sparc/SparcISelLowering.h
+++ b/llvm/lib/Target/Sparc/SparcISelLowering.h
@@ -187,6 +187,8 @@
       return true;
     }
 
+    AtomicExpansionKind shouldExpandAtomicRMWInIR(AtomicRMWInst *AI) const override;
+
     void ReplaceNodeResults(SDNode *N,
                             SmallVectorImpl<SDValue>& Results,
                             SelectionDAG &DAG) const override;