isl_union_map.c: un_op: implement in terms of cond_un_op and rename to total

isl_union_map.c: un_op: implement in terms of cond_un_op and rename to total

un_op is used for functions that can be applied in-place, provided
there is only a single reference.  The original implementation of un_op
makes sure there is a single reference and then applies the transformation
in-place.  Since cond_un_op supports both in-place modification and
creating a new union map, they can both be exploited by un_op depending
on whether the original union map only had one reference.
That is, instead of calling isl_union_map_cow, cond_un_op checks
whether there is only a single references and, if so, performance
the computation in-place.
The name is changed because the original name did not clearly
reflect the difference with cond_un_op.  Besides, now that cond_un_op
handles all cases, it will be renamed to un_op in the next commit.

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