Fix a rare JitStress assert. (dotnet/coreclr#23116)

Fix a rare JitStress assert. (dotnet/coreclr#23116)

* Fix undefined behaviour when we check `this==nullptr`.

* Fix ifdef comment.

* Fix rare jit stress assert.

flowgraph.cpp: !IsUninitialized(tree) can fail if `UninitializedWord` returns a valid pointer value.

Commit migrated from https://github.com/dotnet/coreclr/commit/24d05f5e6cb905da8d2bf84d0b1525586152c908