The previous algorithm would miss hoist opportunities if a larger group
of candidate inames existed which then in turn did not lead to a viable
hoisted condition.

This removes the 'spurious' conditionals in axpy.

- Flag idempotent instructions.

- Exploit idempotent insns in scheduling, allowing them to be executed
  inside "too many" loops.

- Be more exact in what inames to duplicate in CSE pre-computes.

Implement user control over slab decomposition.
Use CSEs instead of register prefetch.
Remove CodeGenerationState.
Carry CSE replacement info in CodeGenerationMapper.
Carry CCM in ExecutionDomain.
Allow user to specify (unchecked) assumptions on parameters,
  to avoid unnecessary conditionals.
Parse desired operation from strings, extract CSEs.
Support for equality constraints in for loops.