This mostly a mechanical change.
However, the change in assignop (subr.go) is a bug fix.
The code didn’t match the comment,
and the comment was correct.
Nevertheless, this CL passes toolstash -cmp.

The last direct reference to dddBound outside
type.go (in typecheck.go) will go away
in a future CL.

Change-Id: Ifb1691e0a07f906712c18c4a4cd23060807a5da5
Reviewed-on: https://go-review.googlesource.com/21235Reviewed-by: Matthew Dempsky <mdempsky@google.com>