Don't perform subtype checks on structurally equal unions
Don't perform subtype checks on structurally equal unions

Summary: If two union reps are structurally equal the unions must also be equal by definition. This is a very conservative estimate of equality, but it's hard to do better than this without adding new information to the union data structure and thus perturbing the cache.

Reviewed By: avikchaudhuri

Differential Revision: D15501466

fbshipit-source-id: 502e4dd0e8f83f1b0fd3c89f4c6c780ecb39e374

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