[release-branch.go1.26] cmd/compile: fix bloop test

Backport of CL 749660. It isn't a simple cherry pick because
there was a refactor at tip.

Fixes #77838

Change-Id: I761c441e82d41d1a7d7c91313401913bb5454546
Reviewed-on: https://go-review.googlesource.com/c/go/+/751240
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Keith Randall <khr@google.com>