[SelectionDAG] Simplify how we drop poison flags in SimplifyDemandedBits.

[craig.topper]

Summary

As far as I can tell what was happening in the original code is
that the getNode call receives the same operands as the original
node with different SDNodeFlags. The logic inside getNode detects
that the node already exists and intersects the flags into the
existing node and returns it. This results in Op and NewOp for the
TLO.CombineTo call always being the same node.

We may have already called CombineTo as part of the recursive handling.
A second call to CombineTo as we unwind the recursion overwrites
the previous CombineTo. I think this means any time we updated the
poison flags that was the only change that ends up getting made
and we relied on DAGCombiner to revisit and call SimplifyDemandedBits
again. The second time the poison flags wouldn't need to be dropped
and we would keep the CombineTo call from further down the recursion.

If this is all correct, I think we can get away with calling setFlags
(or intersectFlagsWith) to drop the flags and remove the call to
TLO.CombineTo. This way we keep the CombineTo from deeper in the
recursion which should be more efficient.

[spatel]

LGTM - I'm guessing I just copied some nearby code when I added that and never stepped through to see exactly how it proceeded internally; I just saw the asm output was as expected.

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

if (SimplifyDemandedBits(Op0, LoMask, DemandedElts, Known2, TLO,
                              Depth + 1) ||
         // See if the operation should be performed at a smaller bit width.
         ShrinkDemandedOp(Op, BitWidth, DemandedBits, TLO)) {
       if (Flags.hasNoSignedWrap() || Flags.hasNoUnsignedWrap()) {
         // Disable the nsw and nuw flags. We can no longer guarantee that we
         // won't wrap after simplification.
         Flags.setNoSignedWrap(false);
         Flags.setNoUnsignedWrap(false);
-        SDValue NewOp =
-            TLO.DAG.getNode(Op.getOpcode(), dl, VT, Op0, Op1, Flags);
-        return TLO.CombineTo(Op, NewOp);
+        Op->setFlags(Flags);
       }
       return true;
     }
 
     // Attempt to avoid multi-use ops if we don't need anything from them.
     if (!LoMask.isAllOnes() || !DemandedElts.isAllOnes()) {
       SDValue DemandedOp0 = SimplifyMultipleUseDemandedBits(
           Op0, LoMask, DemandedElts, TLO.DAG, Depth + 1);