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

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

This reverts commit cd55f86b8dcfc139ee5c17d32530ac9e758c8bc0 (CL 681937)

Reason for revert: still causing compiler failures on Google test code

Change-Id: I5cd482fd607fd060a523257082d48821b5f965d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/695016
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>