cmd/compile/internal/gc: speed up arith const tests

This change reduces the time to run this test on my machine from

real	0m2.491s
user	0m3.020s
sys	0m0.331s

to

real	0m0.237s
user	0m0.180s
sys	0m0.173s

This will make it reasonable to add more constants to the test.
I am also hopeful that it might help a bit with intermittent
cmd/compile/internal/gc test timeouts on the build dashboard
on the slower builders.

The time savings are entirely in compilation time,
by avoiding generating one giant func main.
Instead, generate tables of tests to be run,
which are translated into static data,
and then loop over those tests.

While we're here, do some minor cleanup:

* Remove the _ssa suffix on function names,
  as that was only needed during ssa bootstrapping.
* Clean up error handling during test generation.
* Make functions single-line, to reduce future diff sizes.
  Diffing giant files is slow.

Change-Id: Ic5fccdb71679169bea756c7d33c07d05e4801860
Reviewed-on: https://go-review.googlesource.com/102956


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>