go/ssa: move build tag guarded tests

Move tests out of files that have build tag guards that
are now true for all supported versions.

Change-Id: Ia8c5c8d20aa11991288a65eab72146adac60a82a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/614457
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: timothy-king