bc: tidying up, no logic changes

bc: tidying up, no logic changes

function                                             old     new   delta
bc_ops_prec_and_assoc                                  -      25     +25
xc_vm_init                                           665     663      -2
bc_parse_ops                                          25       -     -25
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 0/1 up/down: 25/-27)             Total: -2 bytes

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