cmd/compile: simplify constant switch case sorting

cmd/compile: simplify constant switch case sorting

This sort is now only reachable for constant clauses
for a non-interface switch expression value.

Refactor a bit so that the few tests that remain
are concise and easy to read.

Add a test that string length takes priority
over built-in string order.

Change-Id: Iedaa11ff77049d5ad1bf14f54cbb8c3411d589a7
Reviewed-on: https://go-review.googlesource.com/26767
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>