[DAG] visitFP_TO_SINT/FP_TO_UINT - use FoldConstantArithmetic to attempt to constant fold

Don't rely on isConstantFPBuildVectorOrConstantFP followed by getNode() will constant fold - FoldConstantArithmetic will do all of this for us.

Cleanup for #112682

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -18128,8 +18128,8 @@ SDValue DAGCombiner::visitFP_TO_SINT(SDNode *N) {
     return DAG.getUNDEF(VT);
 
   // fold (fp_to_sint c1fp) -> c1
-  if (DAG.isConstantFPBuildVectorOrConstantFP(N0))
-    return DAG.getNode(ISD::FP_TO_SINT, DL, VT, N0);
+  if (SDValue C = DAG.FoldConstantArithmetic(ISD::FP_TO_SINT, DL, VT, {N0}))
+    return C;
 
   return FoldIntToFPToInt(N, DL, DAG);
 }
@@ -18144,8 +18144,8 @@ SDValue DAGCombiner::visitFP_TO_UINT(SDNode *N) {
     return DAG.getUNDEF(VT);
 
   // fold (fp_to_uint c1fp) -> c1
-  if (DAG.isConstantFPBuildVectorOrConstantFP(N0))
-    return DAG.getNode(ISD::FP_TO_UINT, DL, VT, N0);
+  if (SDValue C = DAG.FoldConstantArithmetic(ISD::FP_TO_UINT, DL, VT, {N0}))
+    return C;
 
   return FoldIntToFPToInt(N, DL, DAG);
 }