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.