runtime: speed up ifaceeq for direct ifaces

name                    old time/op  new time/op  delta
EfaceCmpDiff-8           421ns ± 3%   299ns ± 3%  -28.93%  (p=0.000 n=92+94)
EfaceCmpDiffIndirect-8   497ns ± 4%   496ns ± 3%     ~     (p=0.840 n=98+92)

Change-Id: Id1a8c779413ba35ab0f58d055870b6a0714b51b7
Reviewed-on: https://go-review.googlesource.com/c/152163


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>