[DAGCombiner] Don't create sexts of deleted xors when they were in-visit replaced

Fixes a bug introduced by D91589.

When folding `(sext (not i1 x)) -> (add (zext i1 x), -1)`, we try to replace the not first when possible. If we replace the not in-visit, then the now invalidated node will be returned, and subsequently we will return an invalid sext. In cases where the not is replaced in-visit we can simply return SDValue, as the not in the current sext should have already been replaced.

Thanks @jgorbe, for finding the below reproducer.

The following reduced test case crashes clang when built with `clang -O1 -frounding-math`:

```
template <class> class a {
  int b() { return c == 0.0 ? 0 : -1; }
  int c;
};
template class a<long>;
```

A debug build of clang produces this "assertion failed" error:
```
clang: /home/jgorbe/code/llvm/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:264: void {anonymous}::DAGCombiner::AddToWorklist(llvm::
SDNode*): Assertion `N->getOpcode() != ISD::DELETED_NODE && "Deleted Node added to Worklist"' failed.
```

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D93274