[RISCV] Add cost for @llvm.experimental.vp.splat

This is split off from llvm#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.

There's no tests for i1 vectors, since we can't lower vp splats of them yet and currently crash.

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

Author: lukel97

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -1155,6 +1155,11 @@ 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);
+    return LT.first *
+           getRISCVInstructionCost(RISCV::VMV_V_X, LT.second, CostKind);
+  }
   case Intrinsic::vp_reduce_add:
   case Intrinsic::vp_reduce_fadd:
   case Intrinsic::vp_reduce_mul: