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

[Iris Shi via llvm-commits]

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

I've tested locally, nothing changes when adding the removed combines back. But regressions happen when `fold (fabs (fabs x)) -> (fabs x)` at L18878-L18880 is removed.

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