Update/Fix LoopUtils::stmtBodySkew to handle loop step.

- loop step wasn't handled and there wasn't a TODO or an assertion; fix this.
- rename 'delay' to shift for consistency/readability.
- other readability changes.
- remove duplicate attribute print for DmaStartOp; fix misplaced attribute
  print for DmaWaitOp
- add build method for AddFOp (unrelated to this CL, but add it anyway)

PiperOrigin-RevId: 224892958