[dev.typeparams] cmd/compile: allow typecheck of OCHECKNIL

This CL makes OCHECKNIL typecheckable. Simplifies IR construction code
slightly, and gives one convenient place to check for misuse.

Change-Id: I280b8e47eddcac12947a41d6f911b25bc12a66bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/330194


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>