MLStmt cloning and IV replacement for loop unrolling, add constant pool to

MLFunctions.

- MLStmt cloning and IV replacement
- While at this, fix the innermostLoopGatherer to actually gather all the
  innermost loops (it was stopping its walk at the first innermost loop it
  found)
- Improve comments for MLFunction statement classes, fix inheritance order.

- Fixed StmtBlock destructor.

PiperOrigin-RevId: 207049173