cmd/compile: remove 'ext' fields from unified IR reader/writer types

This is a vestigial artifact of how I initially split apart the public
and private data for objects. But now objects are split into more
parts, and it's proven easier to just keep them as separate variables.
So it's time to cleanup the initial public/private code to follow the
same approach.

Change-Id: I3976b19fb433cbe21d299d3799ec616f9e59561e
Reviewed-on: https://go-review.googlesource.com/c/go/+/348412


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>