[RISCV] Add cost for @llvm.experimental.vp.splat (#117313)

This is split off from #115274. There doesn't seem to be an easy way to
share this with getShuffleCost since that requires passing in a real
insert_element operand to get it to recognise it's a scalar splat.

For i1 vectors we can't currently lower them so it returns an invalid
cost.

---------

Co-authored-by: Shih-Po Hung <[email protected]>

Author: lukel97

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -1159,6 +1159,16 @@ RISCVTTIImpl::getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
     return getCmpSelInstrCost(Instruction::Select, ICA.getReturnType(),
                               ICA.getArgTypes()[0], CmpInst::BAD_ICMP_PREDICATE,
                               CostKind);
+  case Intrinsic::experimental_vp_splat: {
+    auto LT = getTypeLegalizationCost(RetTy);
+    // TODO: Lower i1 experimental_vp_splat
+    if (!ST->hasVInstructions() || LT.second.getScalarType() == MVT::i1)
+      return InstructionCost::getInvalid();
+    return LT.first * getRISCVInstructionCost(LT.second.isFloatingPoint()
+                                                  ? RISCV::VFMV_V_F
+                                                  : RISCV::VMV_V_X,
+                                              LT.second, CostKind);
+  }
   case Intrinsic::vp_reduce_add:
   case Intrinsic::vp_reduce_fadd:
   case Intrinsic::vp_reduce_mul: