bc: remove use of "BcInstPtr ip" object from loop parsing

bc: remove use of "BcInstPtr ip" object from loop parsing

function                                             old     new   delta
zbc_parse_stmt_possibly_auto                        1964    1909     -55
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-55)             Total: -55 bytes
   text	   data	    bss	    dec	    hex	filename
 981934	    485	   7296	 989715	  f1a13	busybox_old
 981879	    485	   7296	 989660	  f19dc	busybox_unstripped

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