i965: Don't make consumers of brw_DO()/brw_WHILE() track loop start.

i965: Don't make consumers of brw_DO()/brw_WHILE() track loop start.

This is a similar cleanup to what we did for brw_IF(), brw_ELSE(),
brw_ENDIF() handling.

Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>