cmd/compile: compile cap(ch) as call to runtime.chancap

cmd/compile: compile cap(ch) as call to runtime.chancap

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

For #37196.

Change-Id: I965426d446964b9b4958e4613246002a7660e7eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/568375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>