cmd/compile: discount calls to closure variables

cmd/compile: discount calls to closure variables

This causes more inlining for rangefunc code.
The PAUTOHEAP case catches closure-passed-as-param
to function that returns a closure, that calls the
outer PPARAM.

Change-Id: I927cf2e1924ed2191e0ad8be9a1f2e793b6f2b38
Reviewed-on: https://go-review.googlesource.com/c/go/+/620220
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>