cmd/compile: change ssa test to avoid go run -gcflags=-d=ssa/check/on

cmd/compile: change ssa test to avoid go run -gcflags=-d=ssa/check/on

In the new content-based staleness world, setting -gcflags like this
recompiles all the packages involved in running the program, not just
the "stale" ones. So go run -gcflags=-d=ssa/check/on recompiles
runtime with those flags too, which is not what the test is trying
to check.

Change-Id: I4dbd5bf2970c3a622c01de84bd8aa9d5e9ec5239
Reviewed-on: https://go-review.googlesource.com/74570
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>