cmd/compile: break apart dumptypestructs

cmd/compile: break apart dumptypestructs

dumptypestructs did several different jobs.
Split them into separate functions
and call them in turn.

Hand dumptypestructs a list of dcls,
rather than reading the global.

Rename dumpptabs for (marginal) clarity.

This is groundwork for compiling autogenerated
functions concurrently.

Passes toolstash-check.

Change-Id: I627a1dffc70a7e4b7b4436ab19af1406267f01dc
Reviewed-on: https://go-review.googlesource.com/41501
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>