go/types, types2: factor out isdddArray and inNode helper functions
Preparation for generation of go/types/literals.go from types2 sources.
Change-Id: I9af23fbe1e448976394ddd7b348188c2595d8afe
Reviewed-on: https://go-review.googlesource.com/c/go/+/610557
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Tim King <taking@google.com>
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com>