opencl backend: print max macro definition if and when needed

opencl backend: print max macro definition if and when needed

Commit fd0e714 (opencl backend: avoid uninitialized cl_mem objects,
Wed May 7 12:56:20 2014 +0100) introduced a call to
isl_ast_op_type_print_macro to print a macro definition for
isl_ast_op_max because the max may end up getting used
in allocate_device_array.
Presumably the call was put there to avoid the macro definition
getting printed several times.  Now that macro definitions are
only printed once, the call can be moved to allocate_device_array and
it can be performed only when it is actually needed.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>