cmd/compile/internal/types2: consolidate verification logic

Change an internal call of instantiateLazy to call Instantiate, so that
we can consolidate the logic for invoking verification.

This made verification of signatures lazy, which is not necessary but
should be harmless.

Change-Id: I2e59b04ac859e08c2e2910ded3c183093d1e34a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/342149


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>