ProcArrayGroupClearXid() compare-exchange style

[Noah Misch]

ProcArrayGroupClearXid() has this:

        while (true)
        {
                nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);

                ...

                if 
(pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
                                                                                
   &nextidx,
                                                                                
   (uint32) proc->pgprocno))
                        break;
        }

This, from UnpinBuffer(), is our more-typical style:

                old_buf_state = pg_atomic_read_u32(&buf->state);
                for (;;)
                {
                        ...

                        if (pg_atomic_compare_exchange_u32(&buf->state, 
&old_buf_state,
                                                                                
           buf_state))
                                break;
                }

That is, we typically put the pg_atomic_read_u32() outside the loop.  After
the first iteration, it is redundant with the side effect of
pg_atomic_compare_exchange_u32().  I haven't checked whether this materially
improves performance, but, for style, I would like to change it in HEAD.

diff --git a/src/backend/storage/ipc/procarray.c 
b/src/backend/storage/ipc/procarray.c
index 8abcfdf..3da5307 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -490,15 +490,15 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId 
latestXid)
        /* We should definitely have an XID to clear. */
        Assert(TransactionIdIsValid(allPgXact[proc->pgprocno].xid));
 
        /* Add ourselves to the list of processes needing a group XID clear. */
        proc->procArrayGroupMember = true;
        proc->procArrayGroupMemberXid = latestXid;
+       nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
        while (true)
        {
-               nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
                pg_atomic_write_u32(&proc->procArrayGroupNext, nextidx);
 
                if 
(pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
                                                                                
   &nextidx,
                                                                                
   (uint32) proc->pgprocno))
                        break;