[Polly] Add greedy fusion algorithm.

When the option -polly-loopfusion-greedy is set, the ScheduleOptimizer
tries to aggressively fuse any band it can and does not violate any

As part if the implementation, the functionalty for copying a band
into an new schedule was extracted out of the ScheduleTreeRewriter.

GitOrigin-RevId: 64489255be4903dc8683aff8dade315461a0a397
15 files changed