when merging prop types in object resolution, don't produce intersections of identical types
when merging prop types in object resolution, don't produce intersections of identical types

Summary: No sense in generating intersections of identical types. This will also allow us to determine when a spread of an intersected type is identical to both of the types being intersected.

Reviewed By: jbrown215

Differential Revision: D18535260

fbshipit-source-id: 0729e2f394bb86a64ba787fa927d48b80b50a830

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow