test: re-enable phi optimization test

CL 28978 (6ec993ad) accidentally disabled the test (it would only
run if amd64 AND s390x, whereas it should be amd64 OR s390x).

Change-Id: I23c1ad71724ff55f5808d5896b19b62c8ec5af76
Reviewed-on: https://go-review.googlesource.com/28981Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/phiopt.go

-// +build amd64,s390x
+// +build amd64 s390x
 // errorcheck -0 -d=ssa/phiopt/debug=3
 
 // Copyright 2016 The Go Authors. All rights reserved.