SDAG: Make sure we use an allocatable reg class when we create this vreg

As per the discussion on r280783, if constrainRegClass fails we need
to call getAllocatableClass like we did before that commit.

llvm-svn: 285467

llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp

@@ -338,6 +338,8 @@ InstrEmitter::AddRegisterOperand(MachineInstrBuilder &MIB,
       const TargetRegisterClass *ConstrainedRC
         = MRI->constrainRegClass(VReg, OpRC, MinRCSize);
       if (!ConstrainedRC) {
+        OpRC = TRI->getAllocatableClass(OpRC);
+        assert(OpRC && "Constraints cannot be fulfilled for allocation");
         unsigned NewVReg = MRI->createVirtualRegister(OpRC);
         BuildMI(*MBB, InsertPos, Op.getNode()->getDebugLoc(),
                 TII->get(TargetOpcode::COPY), NewVReg).addReg(VReg);