The prealloc map seems to exist to avoid adding a field to all nodes.
Now we can add a field to just the nodes that need the field,
so let's do that and avoid having a magic global with extra node state
that isn't preserved by operations like Copy nor printed by Dump.

This also makes clear which nodes can be prealloc'ed.
In particular, the code in walkstmt looked up an entry in
prealloc using an ONAME node, but there's no code that
ever stores such an entry, so the lookup never succeeded.
Having fields makes that kind of thing easier to see and fix.

Passes buildall w/ toolstash -cmp.

Change-Id: I418ad0e2847615c08868120c13ee719dc0b2eacb
Reviewed-on: https://go-review.googlesource.com/c/go/+/278915


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>