[dev.typeparams] cmd/compile/internal/types2: move instantiation code to instantiate.go (cleanup)

No code changes besides moving the two functions and updating a
couple of file comments.

Change-Id: I13a6a78b6e8c132c20c7f81a329f31d5edab0453
Reviewed-on: https://go-review.googlesource.com/c/go/+/333589


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>