[release-branch.go1.18] cmd/compile/internal/types2: use correct value of iota

[release-branch.go1.18] cmd/compile/internal/types2: use correct value of iota

Fixes #52441.

Change-Id: I5cbf8c448dba037e9e0c5fe8f209401d6bf7d43f
Reviewed-on: https://go-review.googlesource.com/c/go/+/401134
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/401174
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>