[NFC][GlobalISel] Add check for potentially dereferencing null return value

[Petar.Avramovic]

Is this some advanced compiler warning? It looks that assert can't trigger because of if (!Reg) and getConstantVRegValWithLookThrough already checked that Reg is defined by G_FCONSTANT via isFConstant

[yubing]

yeah i checked the code, you're right. getFConstantVRegValWithLookThrough's isFConstant ensure Reg is defined by TargetOpcode::G_FCONSTANT @Petar.Avramovic

llvm/lib/CodeGen/GlobalISel/Utils.cpp

 }
 
 std::optional<FPValueAndVReg> llvm::getFConstantVRegValWithLookThrough(
     Register VReg, const MachineRegisterInfo &MRI, bool LookThroughInstrs) {
   auto Reg = getConstantVRegValWithLookThrough(
       VReg, MRI, isFConstant, getCImmOrFPImmAsAPInt, LookThroughInstrs);
   if (!Reg)
     return std::nullopt;
-  return FPValueAndVReg{getConstantFPVRegVal(Reg->VReg, MRI)->getValueAPF(),
-                        Reg->VReg};
+  const ConstantFP *CFP = getConstantFPVRegVal(Reg->VReg, MRI);
+  assert(CFP && "invalid ConstantFP!");
+  return FPValueAndVReg{CFP->getValueAPF(), Reg->VReg};
 }
 
 const ConstantFP *
 llvm::getConstantFPVRegVal(Register VReg, const MachineRegisterInfo &MRI) {
   MachineInstr *MI = MRI.getVRegDef(VReg);
   if (TargetOpcode::G_FCONSTANT != MI->getOpcode())
     return nullptr;
   return MI->getOperand(1).getFPImm();