cmd/compile: cleanup test code pre Unified IR

cmd/compile: cleanup test code pre Unified IR

Change-Id: Ibb9a7ce37ac28873aa720a64d1f89cb87fc5fa27
Reviewed-on: https://go-review.googlesource.com/c/go/+/543315
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Tim King <taking@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>