cmd/compile: compile len(ch) as call to runtime.chanlen

cmd/compile: compile len(ch) as call to runtime.chanlen

An upcoming CL will give this call more to do.
For now, separate out the compiler change that
stops inlining the computation.

Change-Id: I4c5cbd84a0694b306191bff38cc6ea2d69458d7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/564556
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>