[SelectionDAG] Do not build illegal nodes with users (#108573)

When we build a node with illegal type which has a user, it's possible
that it can end up being processed by the DAG combiner later before it's
removed, which can trigger an assert expecting the types to be legalized
already.

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -6799,14 +6799,17 @@ SDValue SelectionDAG::FoldConstantArithmetic(unsigned Opcode, const SDLoc &DL,
     // Constant fold the scalar operands.
     SDValue ScalarResult = getNode(Opcode, DL, SVT, ScalarOps, Flags);
 
-    // Legalize the (integer) scalar constant if necessary.
-    if (LegalSVT != SVT)
-      ScalarResult = getNode(ExtendCode, DL, LegalSVT, ScalarResult);
-
     // Scalar folding only succeeded if the result is a constant or UNDEF.
     if (!ScalarResult.isUndef() && ScalarResult.getOpcode() != ISD::Constant &&
         ScalarResult.getOpcode() != ISD::ConstantFP)
       return SDValue();
+
+    // Legalize the (integer) scalar constant if necessary. We only do
+    // this once we know the folding succeeded, since otherwise we would
+    // get a node with illegal type which has a user.
+    if (LegalSVT != SVT)
+      ScalarResult = getNode(ExtendCode, DL, LegalSVT, ScalarResult);
+
     ScalarResults.push_back(ScalarResult);
   }