ScheduleOpt: Use band forest to get the schedules

isl introduced a new representation for the schedules it calculates. The new
representation uses a forest of bands and is closer to the structure of the
data as the old interface. Switch to the new interface, as it is nicer to use
and as the old interface will soon be removed from isl.

WARNING: This commit needs a version of isl that is more recent that the one
         included in CLooG. See:
	 http://polly.grosser.es/get_started.html#islTrunk
llvm-svn: 134181