Scheduler: Simplify dependences by default (only isl)

This speeds up the scheduler by orders of magnitude and in addition yields often
to a better schedule.

With this we can compile all polybench kernels with less than 5x compile time
overhead. In general the overhead is even less than 2-3x.  This is still with
running a lot of redundant passes and no compile time tuning at all. There are
several obvious areas where we can improve here further.

There are also two test cases where we cannot find a schedule any more (cholesky
and another). I will look into them later on.

With this we have a very solid base line from which we can start to optimize
further.

llvm-svn: 149263