To refine a set of possibly equivalent values, the old CSE algorithm
picked one value, compared it against all the others, and made two sets
out of the results (the values that match the picked value and the
values that didn't).  Unfortunately, this leads to O(n^2) behavior. The
picked value ends up being equal to no other values, we make size 1 and
size n-1 sets, and then recurse on the size n-1 set.

Instead, sort the set by the equivalence classes of its arguments.  Then
we just look for spots in the sorted list where the equivalence classes
of the arguments change.  This lets us do a multi-way split for O(n lg
n) time.

This change makes cmpDepth unnecessary.

The refinement portion used to call the type comparator.  That is
unnecessary as the type was already part of the initial partition.

Lowers time of 16361 from 8 sec to 3 sec.
Lowers time of 15112 from 282 sec to 20 sec. That's kind of unfair, as
CL 30257 changed it from 21 sec to 282 sec. But that CL fixed other bad
compile times (issue #17127) by large factors, so net still a big win.

Fixes #15112
Fixes #16361

Change-Id: I351ce111bae446608968c6d48710eeb6a3d8e527
Reviewed-on: https://go-review.googlesource.com/30354Reviewed-by: Todd Neal <todd@tneal.org>