[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.

GitOrigin-RevId: 52c30adc7dfe6334b71adf256d81f70e7b976143
10 files changed