[RISCV] Use SelectionDAG::getVScale in lowerVPReverseExperimental. NFCI (#81694)

Use a slightly more idiomatic way of getting vscale. getVScale
performs additional constant folding, but I presume computeKnownBits
also catches these cases too.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -11106,8 +11106,8 @@ RISCVTargetLowering::lowerVPReverseExperimental(SDValue Op,
       // Slide off any elements from past EVL that were reversed into the low
       // elements.
       unsigned MinElts = GatherVT.getVectorMinNumElements();
-      SDValue VLMax = DAG.getNode(ISD::VSCALE, DL, XLenVT,
-                                  DAG.getConstant(MinElts, DL, XLenVT));
+      SDValue VLMax =
+          DAG.getVScale(DL, XLenVT, APInt(XLenVT.getSizeInBits(), MinElts));
       SDValue Diff = DAG.getNode(ISD::SUB, DL, XLenVT, VLMax, EVL);
 
       Result = getVSlidedown(DAG, Subtarget, DL, GatherVT,