[dev.typeparams] cmd/compile/internal/types2: use InstantiateLazy to create...

[dev.typeparams] cmd/compile/internal/types2: use InstantiateLazy to create instance types (cleanup)

This change concentrates the creation is lazily instantiated types
in one place (InstantiateLazy). This should also make it easier to
replace the implementation of lazily instantiated types (e.g. getting
rid of instance types).

Change-Id: I452c463219b466ce79f227c44fb67b79d428842a
Reviewed-on: https://go-review.googlesource.com/c/go/+/333669


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