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.