[X86] combineTruncatedArithmetic - move more of fold inside combinei64TruncSrlConstant

Let combinei64TruncSrlConstant decide when the fold is invalid instead of splitting so many of the conditions with combineTruncatedArithmetic

NOTE: We can probably relax the i32 truncation constraint to <= i32, perform the SRL as i32 and then truncate further.

Noticed while triaging #141496

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -54214,22 +54214,31 @@ static SDValue combineLRINT_LLRINT(SDNode *N, SelectionDAG &DAG,
 // cases.
 static SDValue combinei64TruncSrlConstant(SDValue N, EVT VT, SelectionDAG &DAG,
                                           const SDLoc &DL) {
+  assert(N.getOpcode() == ISD::SRL && "Unknown shift opcode");
+  std::optional<uint64_t> ValidSrlConst = DAG.getValidShiftAmount(N);
+  if (!ValidSrlConst)
+    return SDValue();
+  uint64_t SrlConstVal = *ValidSrlConst;
 
   SDValue Op = N.getOperand(0);
-  APInt OpConst = Op.getConstantOperandAPInt(1);
-  APInt SrlConst = N.getConstantOperandAPInt(1);
-  uint64_t SrlConstVal = SrlConst.getZExtValue();
   unsigned Opcode = Op.getOpcode();
+  assert(VT == MVT::i32 && Op.getValueType() == MVT::i64 &&
+         "Illegal truncation types");
+
+  if ((Opcode != ISD::ADD && Opcode != ISD::OR && Opcode != ISD::XOR) ||
+      !isa<ConstantSDNode>(Op.getOperand(1)))
+    return SDValue();
+  const APInt &OpConst = Op.getConstantOperandAPInt(1);
 
-  if (SrlConst.ule(32) ||
+  if (SrlConstVal <= 32 ||
       (Opcode == ISD::ADD && OpConst.countr_zero() < SrlConstVal))
     return SDValue();
 
   SDValue OpLhsSrl =
       DAG.getNode(ISD::SRL, DL, MVT::i64, Op.getOperand(0), N.getOperand(1));
   SDValue Trunc = DAG.getNode(ISD::TRUNCATE, DL, VT, OpLhsSrl);
 
-  APInt NewOpConstVal = OpConst.lshr(SrlConst).trunc(VT.getSizeInBits());
+  APInt NewOpConstVal = OpConst.lshr(SrlConstVal).trunc(VT.getSizeInBits());
   SDValue NewOpConst = DAG.getConstant(NewOpConstVal, DL, VT);
   SDValue NewOpNode = DAG.getNode(Opcode, DL, VT, Trunc, NewOpConst);
 
@@ -54285,20 +54294,8 @@ static SDValue combineTruncatedArithmetic(SDNode *N, SelectionDAG &DAG,
   if (!Src.hasOneUse())
     return SDValue();
 
-  if (VT == MVT::i32 && SrcVT == MVT::i64 && SrcOpcode == ISD::SRL &&
-      isa<ConstantSDNode>(Src.getOperand(1))) {
-
-    unsigned SrcOpOpcode = Src.getOperand(0).getOpcode();
-    if ((SrcOpOpcode != ISD::ADD && SrcOpOpcode != ISD::OR &&
-         SrcOpOpcode != ISD::XOR) ||
-        !isa<ConstantSDNode>(Src.getOperand(0).getOperand(1)))
-      return SDValue();
-
-    if (SDValue R = combinei64TruncSrlConstant(Src, VT, DAG, DL))
-      return R;
-
-    return SDValue();
-  }
+  if (VT == MVT::i32 && SrcVT == MVT::i64 && SrcOpcode == ISD::SRL)
+    return combinei64TruncSrlConstant(Src, VT, DAG, DL);
 
   if (!VT.isVector())
     return SDValue();