[dev.typeparams] cmd/compile/internal/types2: trigger verification while resolving instance

This is a straight port of CL 335978 with minor adjustements to
white space and an error message.

Change-Id: Icfcb562f75802a119ce5d02427bffecf7e279b2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/338097


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