cmd/compile: enable ssacheck for tests in ssa_test.go
cmd/compile: enable ssacheck for tests in ssa_test.go

I thought SSA check was enabled for those tests, but in fact it
was not. Enable it. So we have SSA check on for at least some
tests on all architectures.

Updates #22499.

Change-Id: I51fcdda3af7faab5aeb33bf46c6db309285ce42c
Reviewed-on: https://go-review.googlesource.com/76024
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

akutz/go,golang/go,akutz/go,cloudflare/go,cloudflare/go,golang/go,christopher-henderson/Go,akutz/go,akutz/go,golang/go,Cofyc/go,Cofyc/go,CAFxX/go,akutz/go,cloudflare/go,cloudflare/go,josharian/go,Cofyc/go,josharian/go,codestation/go,nawawi/go,codestation/go,Cofyc/go,nawawi/go,codestation/go,akutz/go,nawawi/go,josharian/go,golang/go,akutz/go,nawawi/go,CAFxX/go,codestation/go,cloudflare/go,CAFxX/go,nawawi/go,codestation/go,CAFxX/go,josharian/go,CAFxX/go,Cofyc/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,Cofyc/go,cloudflare/go,josharian/go,nawawi/go,golang/go,CAFxX/go,josharian/go,codestation/go,nawawi/go,nawawi/go,Cofyc/go,golang/go,golang/go,josharian/go,codestation/go,golang/go,Cofyc/go,josharian/go,cloudflare/go,codestation/go