[CodeGen] Removes unwanted optimisation for TargetConstantFP

[simveg]

Summary

This 'FIXME' popped up in the development of an out-of-tree backend.
Quick fix, but first llvm upstream patch, therefore I do not have commit rights, so if approved please commit?

• Test is not included as this came up in an out-of-tree backend (if required, please hint on how to test this).

[craig.topper]

LGTM

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

 }
 
 SDValue SelectionDAGLegalize::OptimizeFloatStore(StoreSDNode* ST) {
   if (!ISD::isNormalStore(ST))
     return SDValue();
 
   LLVM_DEBUG(dbgs() << "Optimizing float store operations\n");
   // Turn 'store float 1.0, Ptr' -> 'store int 0x12345678, Ptr'
-  // FIXME: We shouldn't do this for TargetConstantFP's.
   // FIXME: move this to the DAG Combiner!  Note that we can't regress due
   // to phase ordering between legalized code and the dag combiner.  This
   // probably means that we need to integrate dag combiner and legalizer
   // together.
   // We generally can't do this one for long doubles.
   SDValue Chain = ST->getChain();
   SDValue Ptr = ST->getBasePtr();
+  SDValue Value = ST->getValue();
   MachineMemOperand::Flags MMOFlags = ST->getMemOperand()->getFlags();
   AAMDNodes AAInfo = ST->getAAInfo();
   SDLoc dl(ST);
-  if (ConstantFPSDNode *CFP = dyn_cast<ConstantFPSDNode>(ST->getValue())) {
+
+  // Don't optimise TargetConstantFP
+  if (Value.getOpcode() == ISD::TargetConstantFP)
+    return SDValue();
+
+  if (ConstantFPSDNode *CFP = dyn_cast<ConstantFPSDNode>(Value)) {
     if (CFP->getValueType(0) == MVT::f32 &&
         TLI.isTypeLegal(MVT::i32)) {
       SDValue Con = DAG.getConstant(CFP->getValueAPF().
                                       bitcastToAPInt().zextOrTrunc(32),
                                     SDLoc(CFP), MVT::i32);
       return DAG.getStore(Chain, dl, Con, Ptr, ST->getPointerInfo(),
                           ST->getOriginalAlign(), MMOFlags, AAInfo);
     }
if (CFP->getValueType(0) == MVT::f64) {
         Hi = DAG.getStore(Chain, dl, Hi, Ptr,
                           ST->getPointerInfo().getWithOffset(4),
                           ST->getOriginalAlign(), MMOFlags, AAInfo);
 
         return DAG.getNode(ISD::TokenFactor, dl, MVT::Other, Lo, Hi);
       }
     }
   }
-  return SDValue(nullptr, 0);
+  return SDValue();
 }
 
 void SelectionDAGLegalize::LegalizeStoreOps(SDNode *Node) {
   StoreSDNode *ST = cast<StoreSDNode>(Node);
   SDValue Chain = ST->getChain();
   SDValue Ptr = ST->getBasePtr();
   SDLoc dl(Node);