[RISCV] Remove last use of @llvm.experimental.vp.splat in RISCVCodeGenPrepare. NFCI (#170543)

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.
Originally this was to avoid vl toggles inside vectorized loops, but
nowadays this shouldn't be necessary because we have RISCVVLOptimizer.
To preserve the test cases where there's no store with VL, 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.

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();