cmd/compile/internal/gc: make tests run faster

TestAssembly takes 20s on my machine,
which is too slow for normal operation.
Marking as -short has its dangers (#17472),
but hopefully we'll soon have a builder for that.

All the SSA tests are hermetic and not time sensitive
and can thus be run in parallel.
Reduces the cmd/compile/internal/gc test time during
all.bash on my laptop from 42s to 7s.

Updates #17751

Change-Id: Idd876421db23b9fa3475e8a9b3355a5dc92a5a29
Reviewed-on: https://go-review.googlesource.com/32585


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>