cmd/compile: allow VARDEF at top level

This was missed as part of adding a top-level VARDEF
for stack tracing (CL 134156).

Fixes #28055

Change-Id: Id14748dfccb119197d788867d2ec6a3b3c9835cf
Reviewed-on: https://go-review.googlesource.com/c/140304


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>