Simplify is_negative
Simplify is_negative

Summary: [Folly] Simplify `is_negative`.

Reviewed By: stevegury

Differential Revision: D14141289

fbshipit-source-id: 88bfd5923d90c5ac23d431110c0f2dc6c476d868

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly