[RISCV] Simplify 16 bit index handling in lowerVECTOR_REVERSE [nfc]

getRealMaxVLen returns an upper bound on the value of VLEN.  We can use this upper bound (which unless explicitly set at command line is going to result in a e8 MaxVLMax of much greater than 256) instead of explicitly handling the unknown case separately from the bounded by number greater than 256 case.

Note as well that this code already implicitly depends on a capped value for VLEN.  If infinite VLEN were possible, than 16 bit indices wouldn't be enough.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -5640,21 +5640,18 @@ SDValue RISCVTargetLowering::lowerVECTOR_REVERSE(SDValue Op,
   MVT VecVT = Op.getSimpleValueType();
   unsigned EltSize = VecVT.getScalarSizeInBits();
   unsigned MinSize = VecVT.getSizeInBits().getKnownMinValue();
-
-  unsigned MaxVLMAX = 0;
-  unsigned VectorBitsMax = Subtarget.getMaxRVVVectorSizeInBits();
-  if (VectorBitsMax != 0)
-    MaxVLMAX =
-        RISCVTargetLowering::computeVLMAX(VectorBitsMax, EltSize, MinSize);
+  unsigned VectorBitsMax = Subtarget.getRealMaxVLen();
+  unsigned MaxVLMAX =
+    RISCVTargetLowering::computeVLMAX(VectorBitsMax, EltSize, MinSize);
 
   unsigned GatherOpc = RISCVISD::VRGATHER_VV_VL;
   MVT IntVT = VecVT.changeVectorElementTypeToInteger();
 
-  // If this is SEW=8 and VLMAX is unknown or more than 256, we need
+  // If this is SEW=8 and VLMAX is potentially more than 256, we need
   // to use vrgatherei16.vv.
   // TODO: It's also possible to use vrgatherei16.vv for other types to
   // decrease register width for the index calculation.
-  if ((MaxVLMAX == 0 || MaxVLMAX > 256) && EltSize == 8) {
+  if (MaxVLMAX > 256 && EltSize == 8) {
     // If this is LMUL=8, we have to split before can use vrgatherei16.vv.
     // Reverse each half, then reassemble them in reverse order.
     // NOTE: It's also possible that after splitting that VLMAX no longer