[llvm] [SDAG] Combine select into ABD?, for const (PR #173581)

[Simon Pilgrim via llvm-commits]

================
@@ -12360,29 +12360,43 @@ SDValue DAGCombiner::foldSelectToABD(SDValue LHS, SDValue RHS, SDValue True,
 
   if (LegalOperations && !hasOperation(ABDOpc, VT))
     return SDValue();
+  if (!VT.isInteger())
+    return SDValue();
+
+  // Hack to support constants. (sub a, const) becomes (add a, -const)
+  SDValue NegRHS = DAG.getNegative(RHS, SDLoc(RHS), VT),
+          NegLHS = DAG.getNegative(LHS, SDLoc(LHS), VT);
----------------
RKSimon wrote:

This will create nodes every time the fold is attempted. Can you limit this to cases where LHS/RHS are constant and use m_SpecificInt - or we could even create a m_SpecificNeg() ?

https://github.com/llvm/llvm-project/pull/173581