cmd/compile: use descriptors for type assertion runtime calls

cmd/compile: use descriptors for type assertion runtime calls

Mostly a reorganization to make further changes easier.

This reorganization will make it easier to add a cache in front
of the runtime call.

Leave the old code alone for dynamic type assertions (aka generics).

Change-Id: Ia7dcb7aeb1f63baf93584ccd792e8e31510e8aea
Reviewed-on: https://go-review.googlesource.com/c/go/+/529196
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>