[DAG] isGuaranteedNotToBeUndefOrPoisonForTargetNode - add fallback implementation (llvm#86125)

Allow targets to rely on TargetLowering::isGuaranteedNotToBeUndefOrPoisonForTargetNode to test nodes for canCreateUndefOrPoisonForTargetNode + all arguments are isGuaranteedNotToBeUndefOrPoison.

Targets can still perform this themselves for specific special case nodes (e.g. target shuffles).

Matches the fallback in SelectionDAG::isGuaranteedNotToBeUndefOrPoison

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -5042,8 +5042,9 @@ bool SelectionDAG::isGuaranteedNotToBeUndefOrPoison(SDValue Op,
 
   // If Op can't create undef/poison and none of its operands are undef/poison
   // then Op is never undef/poison.
-  // NOTE: TargetNodes should handle this in themselves in
-  // isGuaranteedNotToBeUndefOrPoisonForTargetNode.
+  // NOTE: TargetNodes can handle this in themselves in
+  // isGuaranteedNotToBeUndefOrPoisonForTargetNode or let
+  // TargetLowering::isGuaranteedNotToBeUndefOrPoisonForTargetNode handle it.
   return !canCreateUndefOrPoison(Op, PoisonOnly, /*ConsiderFlags*/ true,
                                  Depth) &&
          all_of(Op->ops(), [&](SDValue V) {

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -3786,7 +3786,15 @@ bool TargetLowering::isGuaranteedNotToBeUndefOrPoisonForTargetNode(
        Op.getOpcode() == ISD::INTRINSIC_VOID) &&
       "Should use isGuaranteedNotToBeUndefOrPoison if you don't know whether Op"
       " is a target node!");
-  return false;
+
+  // If Op can't create undef/poison and none of its operands are undef/poison
+  // then Op is never undef/poison.
+  return !canCreateUndefOrPoisonForTargetNode(Op, DemandedElts, DAG, PoisonOnly,
+                                              /*ConsiderFlags*/ true, Depth) &&
+         all_of(Op->ops(), [&](SDValue V) {
+           return DAG.isGuaranteedNotToBeUndefOrPoison(V, PoisonOnly,
+                                                       Depth + 1);
+         });
 }
 
 bool TargetLowering::canCreateUndefOrPoisonForTargetNode(