[FIRRTL] Inliner: Support for ops with regions.

inlineInstances/flattenInstances:
* Walk entire body, not only top-level operations.
  Fixes missing instances and allows inlining them
  when conservatively legal.
* Reject inlining instances under when/match.

inlineInto/flattenInto:
  Walk entire body using new `inliningWalk` method
  that drives the per-operations handling but also
  handles cloning "structure" operations that have
  regions (when/match/layer) and managing what
  should be cloned where.

  This allows inlining modules that contain these
  operations.

Inliner now may produce errors, thread throughout.

This allows the inliner to run earlier in the pipeline,
particularly before LowerLayers.

Author: dtzSiFive