cmd/compile/internal/devirtualize: fix OCONVNOP assertion

cmd/compile/internal/devirtualize: fix OCONVNOP assertion

Fixes #75863

Change-Id: I1e5a0f3880dcd5f820a5b6f4540c49b16a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/711141
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>