Most types are reflexive (k == k for all k of type t), so don't
bother calling equal(k, k) when the key type is reflexive.

Change-Id: Ia716b4198b8b298687843b94b878dbc5e8fc2c65
Reviewed-on: https://go-review.googlesource.com/1480Reviewed-by: Russ Cox <rsc@golang.org>