[dev.simd] cmd/compile: fix unstable output

[dev.simd] cmd/compile: fix unstable output

This CL fixed an error left by CL 718160.

Change-Id: I442ea59bc1ff0dda2914d1858dd5ebe93e2818dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/720281
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>