[AMDGPU] Fix -Wunused-variable when -DLLVM_ENABLE_ASSERTIONS=off

llvm-svn: 340868
diff --git a/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
index 64654de..eed9a4d 100644
--- a/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
+++ b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
@@ -355,8 +355,7 @@
             !TII->getRegisterInfo().isVGPR(MRI, OtherOp.getReg()))
           return false;
 
-        const MachineOperand &SDst = MI->getOperand(1);
-        assert(SDst.isDef());
+        assert(MI->getOperand(1).isDef());
 
         int Op32 =  AMDGPU::getVOPe32(Opc);
         FoldList.push_back(FoldCandidate(MI, CommuteOpNo, OpToFold, true,