[RISCV] Add DAG combine to turn (sub (shl X, 8-Y), (shr X, Y)) into orc.b

This patch generalizes the DAG combine for (sub (shl X, 8), X) => (orc.b X)
into the more general form of (sub (shl X, 8 - Y), (srl X, Y)) => (orc.b X).

Alive2 generalized proof: https://alive2.llvm.org/ce/z/dFcf_n
Related issue: #96595
Related PR: #96680

Author: damokeev

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -13572,8 +13572,10 @@ static SDValue combineSubOfBoolean(SDNode *N, SelectionDAG &DAG) {
   return DAG.getNode(ISD::ADD, DL, VT, NewLHS, NewRHS);
 }
 
-// Looks for (sub (shl X, 8), X) where only bits 8, 16, 24, 32, etc. of X are
-// non-zero. Replace with orc.b.
+// Looks for (sub (shl X, 8-Y), (shr X, Y)) where the Y-th bit in each byte is
+// potentially set. It is fine for Y to be 0, meaning that (sub (shl X, 8), X)
+// is also valid. Replace with (orc.b X). For example, 0b0000_1000_0000_1000 is
+// valid with Y=3, while 0b0000_1000_0000_0100 is not.
 static SDValue combineSubShiftToOrcB(SDNode *N, SelectionDAG &DAG,
                                      const RISCVSubtarget &Subtarget) {
   if (!Subtarget.hasStdExtZbb())
@@ -13587,18 +13589,44 @@ static SDValue combineSubShiftToOrcB(SDNode *N, SelectionDAG &DAG,
   SDValue N0 = N->getOperand(0);
   SDValue N1 = N->getOperand(1);
 
-  if (N0.getOpcode() != ISD::SHL || N0.getOperand(0) != N1 || !N0.hasOneUse())
+  if (N0->getOpcode() != ISD::SHL)
+    return SDValue();
+  
+  auto *ShAmtCLeft = dyn_cast<ConstantSDNode>(N0.getOperand(1));
+  if (!ShAmtCLeft)
+    return SDValue();
+  unsigned ShiftedAmount = 8 - ShAmtCLeft->getZExtValue();
+
+  if (ShiftedAmount >= 8)
+    return SDValue();
+
+  SDValue LeftShiftOperand = N0->getOperand(0);
+  SDValue RightShiftOperand = N1;
+
+  if (ShiftedAmount != 0) { // Right operand must be a right shift.
+    if (N1->getOpcode() != ISD::SRL)
+      return SDValue();
+    auto *ShAmtCRight = dyn_cast<ConstantSDNode>(N1.getOperand(1));
+    if (!ShAmtCRight || ShAmtCRight->getZExtValue() != ShiftedAmount)
+      return SDValue();
+    RightShiftOperand = N1.getOperand(0);
+  }
+
+  // At least one shift should have a single use.
+  if (!N0.hasOneUse() && (ShiftedAmount == 0 || !N1.hasOneUse()))
     return SDValue();
 
-  auto *ShAmtC = dyn_cast<ConstantSDNode>(N0.getOperand(1));
-  if (!ShAmtC || ShAmtC->getZExtValue() != 8)
+  if (LeftShiftOperand != RightShiftOperand)
     return SDValue();
 
-  APInt Mask = APInt::getSplat(VT.getSizeInBits(), APInt(8, 0xfe));
-  if (!DAG.MaskedValueIsZero(N1, Mask))
+  APInt Mask = APInt::getSplat(VT.getSizeInBits(), APInt(8, 0x1));
+  Mask <<= ShiftedAmount;
+  // Check that X has indeed the right shape (only the Y-th bit can be set in
+  // every byte).
+  if (!DAG.MaskedValueIsZero(LeftShiftOperand, ~Mask))
     return SDValue();
 
-  return DAG.getNode(RISCVISD::ORC_B, SDLoc(N), VT, N1);
+  return DAG.getNode(RISCVISD::ORC_B, SDLoc(N), VT, LeftShiftOperand);
 }
 
 static SDValue performSUBCombine(SDNode *N, SelectionDAG &DAG,