[RISCV][TTI] Implement instruction cost for vp_merge. (#112327)

This patch implement the instruction for `vp_merge`, which will generate
similar instruction sequence to the `select` instruction.

Author: ElvisWang123

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -1130,6 +1130,10 @@ RISCVTTIImpl::getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
     return getCmpSelInstrCost(*FOp, ICA.getReturnType(), ICA.getArgTypes()[0],
                               CmpInst::BAD_ICMP_PREDICATE, CostKind);
   }
+  case Intrinsic::vp_merge:
+    return getCmpSelInstrCost(Instruction::Select, ICA.getReturnType(),
+                              ICA.getArgTypes()[0], CmpInst::BAD_ICMP_PREDICATE,
+                              CostKind);
   }
 
   if (ST->hasVInstructions() && RetTy->isVectorTy()) {
@@ -2429,4 +2433,4 @@ bool RISCVTTIImpl::isProfitableToSinkOperands(
     Ops.push_back(&OpIdx.value());
   }
   return true;
-}
+}