cmd/compile: don't instrument counter globals in internal/fuzz

cmd/compile: don't instrument counter globals in internal/fuzz

Fixes: #72766
Change-Id: I45b521e53c2a11e259dc99e2dfc8e40cac39139a
Reviewed-on: https://go-review.googlesource.com/c/go/+/673575
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>