[RISCV] Revise interface of isLegalBitRotate [nfc]

Remove a dead parameter (DAG), and replace the ShuffleVectorSDNode param
with the two things we need from the shuffle (mask and VT).  There's
further room to improve this code, but this gets me what I need for an
upcoming patch.

Author: preames

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -5072,21 +5072,17 @@ static SDValue lowerBitreverseShuffle(ShuffleVectorSDNode *SVN,
   return Res;
 }
 
-static bool isLegalBitRotate(ShuffleVectorSDNode *SVN,
-                             SelectionDAG &DAG,
+static bool isLegalBitRotate(ArrayRef<int> Mask, EVT VT,
                              const RISCVSubtarget &Subtarget,
                              MVT &RotateVT, unsigned &RotateAmt) {
-  SDLoc DL(SVN);
-
-  EVT VT = SVN->getValueType(0);
   unsigned NumElts = VT.getVectorNumElements();
   unsigned EltSizeInBits = VT.getScalarSizeInBits();
   unsigned NumSubElts;
-  if (!ShuffleVectorInst::isBitRotateMask(SVN->getMask(), EltSizeInBits, 2,
+  if (!ShuffleVectorInst::isBitRotateMask(Mask, EltSizeInBits, 2,
                                           NumElts, NumSubElts, RotateAmt))
     return false;
   RotateVT = MVT::getVectorVT(MVT::getIntegerVT(EltSizeInBits * NumSubElts),
-                                  NumElts / NumSubElts);
+                              NumElts / NumSubElts);
 
   // We might have a RotateVT that isn't legal, e.g. v4i64 on zve32x.
   return Subtarget.getTargetLowering()->isTypeLegal(RotateVT);
@@ -5103,7 +5099,7 @@ static SDValue lowerVECTOR_SHUFFLEAsRotate(ShuffleVectorSDNode *SVN,
   EVT VT = SVN->getValueType(0);
   unsigned RotateAmt;
   MVT RotateVT;
-  if (!isLegalBitRotate(SVN, DAG, Subtarget, RotateVT, RotateAmt))
+  if (!isLegalBitRotate(SVN->getMask(), VT, Subtarget, RotateVT, RotateAmt))
     return SDValue();
 
   SDValue Op = DAG.getBitcast(RotateVT, SVN->getOperand(0));
@@ -5142,7 +5138,7 @@ static SDValue lowerShuffleViaVRegSplitting(ShuffleVectorSDNode *SVN,
   // expansion for.
   unsigned RotateAmt;
   MVT RotateVT;
-  if (isLegalBitRotate(SVN, DAG, Subtarget, RotateVT, RotateAmt))
+  if (isLegalBitRotate(Mask, VT, Subtarget, RotateVT, RotateAmt))
     return SDValue();
 
   MVT ElemVT = VT.getVectorElementType();