[RISCV] Remove last use of experimental.vp.splat in RISCVCodeGenPrepare. NFCI

Stacked on #170539

RISCVCodeGenPrepare is the last user of the vp.splat intrinsic, where it uses it to expand a zero strided load into a scalar load and splat. This replaces it with a regular splat followed by a vp_merge to set the lanes past EVL as poison. We need to set the EVL here because RISCVISelDAGToDAG will try and recombine it back into a zero strided load, and we want to preserve the original VL.

We need to set

Author: lukel97

llvm/lib/Target/RISCV/RISCVCodeGenPrepare.cpp

@@ -271,8 +271,10 @@ bool RISCVCodeGenPrepare::expandVPStrideLoad(IntrinsicInst &II) {
   IRBuilder<> Builder(&II);
   Type *STy = VTy->getElementType();
   Value *Val = Builder.CreateLoad(STy, BasePtr);
-  Value *Res = Builder.CreateIntrinsic(Intrinsic::experimental_vp_splat, {VTy},
-                                       {Val, II.getOperand(2), VL});
+  Value *Res = Builder.CreateIntrinsic(
+      Intrinsic::vp_merge, VTy,
+      {II.getOperand(2), Builder.CreateVectorSplat(VTy->getElementCount(), Val),
+       PoisonValue::get(VTy), VL});
 
   II.replaceAllUsesWith(Res);
   II.eraseFromParent();