cmd/compile: allow multi-field structs to be stored directly in interfaces

cmd/compile: allow multi-field structs to be stored directly in interfaces

If the struct is a bunch of 0-sized fields and one pointer field.

Fixes #74092

Change-Id: I87c5d162c8c9fdba812420d7f9d21de97295b62c
Reviewed-on: https://go-review.googlesource.com/c/go/+/681937
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>