[SelectionDAG] Merge constant SDNode arithmetic into foldConstantArithmetic

This is the second patch as part of https://bugs.llvm.org/show_bug.cgi?id=36544

Merging in the ConstantSDNode variant of FoldConstantArithmetic. After this, I will begin merging in FoldConstantVectorArithmetic

I've ensured this patch can build & pass all lit tests in Windows and Linux environments.

Patch by @justice_adams (Justice Adams)

Differential Revision: https://reviews.llvm.org/D74881

Author: RKSimon

llvm/include/llvm/CodeGen/SelectionDAG.h

@@ -1503,10 +1503,6 @@ class SelectionDAG {
   SDValue FoldConstantArithmetic(unsigned Opcode, const SDLoc &DL, EVT VT,
                                  ArrayRef<SDValue> Ops);
 
-  SDValue FoldConstantArithmetic(unsigned Opcode, const SDLoc &DL, EVT VT,
-                                 const ConstantSDNode *C1,
-                                 const ConstantSDNode *C2);
-
   SDValue FoldConstantVectorArithmetic(unsigned Opcode, const SDLoc &DL, EVT VT,
                                        ArrayRef<SDValue> Ops,
                                        const SDNodeFlags Flags = SDNodeFlags());

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -4820,17 +4820,6 @@ static llvm::Optional<APInt> FoldValue(unsigned Opcode, const APInt &C1,
   return llvm::None;
 }
 
-SDValue SelectionDAG::FoldConstantArithmetic(unsigned Opcode, const SDLoc &DL,
-                                             EVT VT, const ConstantSDNode *C1,
-                                             const ConstantSDNode *C2) {
-  if (C1->isOpaque() || C2->isOpaque())
-    return SDValue();
-  if (Optional<APInt> Folded =
-          FoldValue(Opcode, C1->getAPIntValue(), C2->getAPIntValue()))
-    return getConstant(Folded.getValue(), DL, VT);
-  return SDValue();
-}
-
 SDValue SelectionDAG::FoldSymbolOffset(unsigned Opcode, EVT VT,
                                        const GlobalAddressSDNode *GA,
                                        const SDNode *N2) {
@@ -4899,7 +4888,15 @@ SDValue SelectionDAG::FoldConstantArithmetic(unsigned Opcode, const SDLoc &DL,
   // Handle the case of two scalars.
   if (auto *C1 = dyn_cast<ConstantSDNode>(N1)) {
     if (auto *C2 = dyn_cast<ConstantSDNode>(N2)) {
-      SDValue Folded = FoldConstantArithmetic(Opcode, DL, VT, C1, C2);
+      if (C1->isOpaque() || C2->isOpaque())
+        return SDValue();
+
+      Optional<APInt> FoldAttempt =
+          FoldValue(Opcode, C1->getAPIntValue(), C2->getAPIntValue());
+      if (!FoldAttempt)
+        return SDValue();
+
+      SDValue Folded = getConstant(FoldAttempt.getValue(), DL, VT);
       assert((!Folded || !VT.isVector()) &&
              "Can't fold vectors ops with scalar operands");
       return Folded;