cmd/compile: simplify exporting OTYPE nodes

cmd/compile: simplify exporting OTYPE nodes

We only export packages that typechecked successfully, and OTYPE nodes
will always have their Type field set.

Changes the package export format, but only in the compiler-specific
section. No version bump necessary.

Change-Id: I722f5827e73948fceb0432bc8b3b22471fea8f61
Reviewed-on: https://go-review.googlesource.com/63273
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>