The type switch in walkexpr is giant.
Shrink it a little by coalescing identical cases
and removing some vertical whitespace.

No functional changes.

Passes toolstash-check.

Change-Id: I7f7efb4faae1f8657dfafac04585172f99d8b37d
Reviewed-on: https://go-review.googlesource.com/38652
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>