[RISCV][TTI] Fix potential crash of using dyn_cast() in getIntrinsicInstrCost() NFC. (llvm#109379)

This patch fix the potential crash about using dyn_cast in `vp_cmp`
which is same as llvm#109313.

Check if the IntrinsicCostAttrubute contains underlying instruction
first and cast to the VPCmpIntrinsic.

Author: ElvisWang123

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -1039,12 +1039,12 @@ RISCVTTIImpl::getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
   case Intrinsic::vp_fcmp: {
     Intrinsic::ID IID = ICA.getID();
     std::optional<unsigned> FOp = VPIntrinsic::getFunctionalOpcodeForVP(IID);
-    auto *UI = dyn_cast<VPCmpIntrinsic>(ICA.getInst());
-
     // We can only handle vp_cmp intrinsics with underlying instructions.
-    if (!UI)
+    if (!ICA.getInst())
       break;
+
     assert(FOp);
+    auto *UI = cast<VPCmpIntrinsic>(ICA.getInst());
     return getCmpSelInstrCost(*FOp, ICA.getArgTypes()[0], ICA.getReturnType(),
                               UI->getPredicate(), CostKind);
   }