Clean up code that does interface equality. Avoid doing checks
in efaceeq/ifaceeq that we already did before calling those routines.

No noticeable performance changes for existing benchmarks.

name            old time/op  new time/op  delta
EfaceCmpDiff-8   604ns ± 1%   553ns ± 1%  -8.41%  (p=0.000 n=9+10)

Fixes #18618

Change-Id: I3bd46db82b96494873045bc3300c56400bc582eb
Reviewed-on: https://go-review.googlesource.com/38606
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>