isl_pw_*_coalesce: sort function value expressions and always coalesce domains

isl_pw_*_coalesce: sort function value expressions and always coalesce domains

isl_pw_*_coalesce would look for pieces with the same function value
expressions by comparing all pairs of entries.  Sort the pieces
based on the function value expressions instead such that only
successive pieces need to be checked.  In fact, isl_pw_*_sort
already does this check internally.
Also, isl_pw_*_coalesce would only coalesce the domains of pieces
that have been combined.  Coalesce all domains instead.

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