[release-branch.go1.18] cmd/compile: always write fun[0] in incomplete itab

[release-branch.go1.18] cmd/compile: always write fun[0] in incomplete itab

runtime.getitab need filled fun[0] to identify whether
implemented the interface.

Fixes #51738
Fixes #52244

Change-Id: I0173b98f4e1b45e3a0183a5b60229d289140d1e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/399058
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/399974
Reviewed-by: Austin Clements <austin@google.com>