Simplify multixact freezing a bit

Testing for abortedness of a multixact member that's being frozen is
unnecessary: we only need to know whether the transaction is still in
progress or committed to determine whether it must be kept or not.  This
let us simplify the code a bit and avoid a useless TransactionIdDidAbort
test.

Suggested by Andres Freund awhile back.
Avoid uselessly looking up old LOCK_ONLY multixacts

Commit 0ac5ad5134f2 removed an optimization in multixact.c that skipped
fetching members of MultiXactId that were older than our
OldestVisibleMXactId value.  The reason this was removed is that it is
possible for multixacts that contain updates to be older than that
value.  However, if the caller is certain that the multi does not
contain an update (because the infomask bits say so), it can pass this
info down to GetMultiXactIdMembers, enabling it to use the old
optimization.

Pointed out by Andres Freund in 20131121200517.GM7240@alap2.anarazel.de