[llvm] [SelectionDAG] Propagate poison in getNode with two operands if the input is poison. (PR #135387)

[Amy Kwan via llvm-commits]

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@@ -7573,15 +7573,18 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
     } else {
       switch (Opcode) {
       case ISD::SUB:
-        return getUNDEF(VT);     // fold op(undef, arg2) -> undef
+        // fold op(undef, arg2) -> undef, fold op(poison, arg2) ->poison.
+        return N1.getOpcode() == ISD::POISON ? getPOISON(VT) : getUNDEF(VT);
       case ISD::SIGN_EXTEND_INREG:
       case ISD::UDIV:
       case ISD::SDIV:
       case ISD::UREM:
       case ISD::SREM:
       case ISD::SSUBSAT:
       case ISD::USUBSAT:
-        return getConstant(0, DL, VT);    // fold op(undef, arg2) -> 0
+        // fold op(undef, arg2) -> 0, fold op(poison, arg2) ->poison.
----------------
amy-kwan wrote:

Comment nit.
```suggestion
        // fold op(undef, arg2) -> 0, fold op(poison, arg2) -> poison.
```

https://github.com/llvm/llvm-project/pull/135387