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

@@ -3174,11 +3174,11 @@ BackendXidGetPid(TransactionId xid)
 
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
-		int			pgprocno = arrayP->pgprocnos[index];
-		PGPROC	   *proc = &allProcs[pgprocno];
-
 		if (other_xids[index] == xid)
 		{
+			int			pgprocno = arrayP->pgprocnos[index];
+			PGPROC	   *proc = &allProcs[pgprocno];
+
 			result = proc->pid;
 			break;
 		}