[AMDGPU] Added assert in SIFoldOperands before ptr use. NFC.

Author: rampitec

llvm/lib/Target/AMDGPU/SIFoldOperands.cpp

@@ -1208,6 +1208,7 @@ void SIFoldOperands::foldInstOperand(MachineInstr &MI,
     Copy->addImplicitDefUseOperands(*MF);
 
   for (FoldCandidate &Fold : FoldList) {
+    assert(!Fold.isReg() || Fold.OpToFold);
     if (Fold.isReg() && Register::isVirtualRegister(Fold.OpToFold->getReg())) {
       Register Reg = Fold.OpToFold->getReg();
       MachineInstr *DefMI = Fold.OpToFold->getParent();