[Polly] Implement user-directed loop distribution/fission.

This is a simple version without the possibility to define distribute
points or followup-transformations. However, it is the first
transformation that has to check whether the transformation is correct.

It interprets the same metadata as the LoopDistribute pass.

Re-apply after revert in c7bcd72a with
fix: Take isBand out of #ifndef NDEBUG since it now is used
unconditionally.