cmd/compile: remove ignored bool from exported ODCL nodes

cmd/compile: remove ignored bool from exported ODCL nodes

This shortens the export format by 1 byte for each exported ODCL
node in inlined function bodies.

Maintain backward compatibility by updating format version and
continue to accept older format.

Change-Id: I549bb3ade90bc0f146decf8016d5c9c3f14eb293
Reviewed-on: https://go-review.googlesource.com/27999
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>