bc: simplify use of "ip" in loop parsing functions

bc: simplify use of "ip" in loop parsing functions

function                                             old     new   delta
zbc_parse_stmt_possibly_auto                        2106    2065     -41
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-41)             Total: -41 bytes
   text	   data	    bss	    dec	    hex	filename
 982076	    485	   7296	 989857	  f1aa1	busybox_old
 982035	    485	   7296	 989816	  f1a78	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>