[RISCV] Make single source reverse legal in isShuffleMaskLegal (llvm#125949)

This enables DAG combines to form this mask. Reverse is generally linear
in LMUL so this is reasonable, and results in better codegen for the 2
source variants.

For <= m1, the change is only slightly profitable if at all. We trade
some mask creation and an extract vrsub for a vslideup.vi. This is
likely roughly neutral. At >= m2, this is distinctly profitable as
generic DAG pushes the reverse into the two operands. We effectively
already did this for one operand, but the other was hitting a full
O(LMUL^2) shuffle. Moving that to be O(LMUL/2) operation is a big win.

Author: preames

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -5815,14 +5815,16 @@ bool RISCVTargetLowering::isShuffleMaskLegal(ArrayRef<int> M, EVT VT) const {
   if (ShuffleVectorSDNode::isSplatMask(M.data(), VT))
     return true;
 
+  const unsigned NumElts = M.size();
   MVT SVT = VT.getSimpleVT();
 
   // Not for i1 vectors.
   if (SVT.getScalarType() == MVT::i1)
     return false;
 
   int Dummy1, Dummy2;
-  return (isElementRotate(Dummy1, Dummy2, M) > 0) ||
+  return ShuffleVectorInst::isReverseMask(M, NumElts) ||
+         (isElementRotate(Dummy1, Dummy2, M) > 0) ||
          isInterleaveShuffle(M, SVT, Dummy1, Dummy2, Subtarget);
 }