[llvm] [SelectionDAG] Handle `fneg`/`fabs`/`fcopysign` in `SimplifyDemandedBits` (PR #139239)

[Matt Arsenault via llvm-commits]

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@@ -18259,21 +18259,6 @@ SDValue DAGCombiner::visitFCOPYSIGN(SDNode *N) {
     }
   }
 
-  // copysign(fabs(x), y) -> copysign(x, y)
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arsenm wrote:

I think it may help if we can observe test changes with the SimplifyDemandedBits changes alone, without dropping the existing combines

https://github.com/llvm/llvm-project/pull/139239