Use CompactAttribute more often, when possible

5983a4cff added CompactAttribute to TupleDesc to allow commonly
accessed fields from the FormData_pg_attribute array to be accessed
more quickly by having to load fewer cachelines from memory. That
commit also went around and changed many locations to use
CompactAttribute, but not all locations.

The attached aims to adjust the remaining locations that I'm
comfortable changing. I've avoided doing anything in tablecmds.c as
that's where TupleDescs tend to get changed and I think there's a bit
of undue risk using CompactAttribute there, just in case it gets
accessed after the TupleDesc is changed and before flushing the
pending changes to CompactAttribute in populate_compact_attribute().

Also, I'm aware that the code around the population of the
CompactAttribute has changed a bit since it was added. attgenerated no
longer mirrors its equivalent pg_attribute column. If that was made to
mirror that column again, then there are a few extra locations that
could be made to use CompactAttribute. There's also some more
complexity around CompactAttribute.attnotnull that's crept in. I think
I roughly understand the need for that, but the intent of
CompactAttribute mirroring commonly used pg_attribute fields is made
no longer true by those changes as it now contains extra information
that's unavailable in pg_attribute. It'd be nice if pg_attribute also
had the various additional states of attnotnull that CompactAttribute
now has. </rant>

I don't think the attached is very interesting to look at, but l'll
leave it here for a bit just in case anyone wants to look. Otherwise,
I plan to just treat it as a follow-up of 5983a4cff.

David