cmd/compile: allow StructSelect [x] of interface data fields for x>0

cmd/compile: allow StructSelect [x] of interface data fields for x>0

As of CL 681937 we can now have structs which are pointer shaped, but
their pointer field is not the first field, like struct{ struct{}; *int }.

Fixes #74888

Change-Id: Idc80f6b1abde3ae01437e2a9cadb5aa23d04b806
Reviewed-on: https://go-review.googlesource.com/c/go/+/693415
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>