[DAGCombine] Fix unchecked calls to DAGCombiner::*ExtPromoteOperand

Other calls to DAGCombiner::*PromoteOperand check the result, but here it could cause an assertion in getNode. 
Falling back to any extend in this case instead of failing outright seems correct to me.

No test case because:
The failure was triggered by an out of tree backend. In order to trigger it, a backend would need to overload 
TargetLowering::IsDesirableToPromoteOp to return true for a type for which ISD::SIGN_EXTEND_INREG is marked 
illegal. In tree, only X86 overloads and sometimes returns true for MVT::i16 yet it marks 
setOperationAction(ISD::SIGN_EXTEND_INREG, MVT::i16  , Legal);.

Patch by Jacob Young!

Differential Revision: https://reviews.llvm.org/D33633

llvm-svn: 304723

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -1028,13 +1028,13 @@ SDValue DAGCombiner::PromoteOperand(SDValue Op, EVT PVT, bool &Replace) {
   switch (Opc) {
   default: break;
   case ISD::AssertSext:
-    return DAG.getNode(ISD::AssertSext, DL, PVT,
-                       SExtPromoteOperand(Op.getOperand(0), PVT),
-                       Op.getOperand(1));
+    if (SDValue Op0 = SExtPromoteOperand(Op.getOperand(0), PVT))
+      return DAG.getNode(ISD::AssertSext, DL, PVT, Op0, Op.getOperand(1));
+    break;
   case ISD::AssertZext:
-    return DAG.getNode(ISD::AssertZext, DL, PVT,
-                       ZExtPromoteOperand(Op.getOperand(0), PVT),
-                       Op.getOperand(1));
+    if (SDValue Op0 = ZExtPromoteOperand(Op.getOperand(0), PVT))
+      return DAG.getNode(ISD::AssertZext, DL, PVT, Op0, Op.getOperand(1));
+    break;
   case ISD::Constant: {
     unsigned ExtOpc =
       Op.getValueType().isByteSized() ? ISD::SIGN_EXTEND : ISD::ZERO_EXTEND;