Improve BackendXidGetPid() to only access allProcs on matching XID

Compilers are able to optimize that, but it makes the code slightly more
readable this way.

Author: Zhao Junwang
Reviewed-by: Ashutosh Bapat
Discussion: https://postgr.es/m/CAEG8a3+i9gtqF65B+g_puVaCQuf0rZC-EMqMyEjGFJYOqUUWfA@mail.gmail.com

src/backend/storage/ipc/procarray.c

@@ -3173,12 +3173,12 @@ BackendXidGetPid(TransactionId xid)
 	LWLockAcquire(ProcArrayLock, LW_SHARED);
 
 	for (index = 0; index < arrayP->numProcs; index++)
+	{
+		if (other_xids[index] == xid)
 		{
 			int			pgprocno = arrayP->pgprocnos[index];
 			PGPROC	   *proc = &allProcs[pgprocno];
 
-		if (other_xids[index] == xid)
-		{
 			result = proc->pid;
 			break;
 		}