Extend loop unroll/unroll-and-jam to affine bounds + refactor related code.

- extend loop unroll-jam similar to loop unroll for affine bounds
- extend both loop unroll/unroll-jam to deal with cleanup loop for non multiple
  of unroll factor.
- extend promotion of single iteration loops to work with affine bounds
- fix typo bugs in loop unroll
- refactor common code b/w loop unroll and loop unroll-jam
- move prototypes of non-pass transforms to LoopUtils.h
- add additional builder methods.
- introduce loopUnrollUpTo(factor) to unroll by either factor or trip count,
  whichever is less.
- remove Statement::isInnermost (not used for now - will come back at the right
  place/in right form later)

PiperOrigin-RevId: 213471227