Better typing and type verification

- Extend verifier to check types of JS and Simplified nodes.
- Untyped nodes now contain NULL as types, enforcing hard failure.
- Typer immediately installs itself as a decorator; remove explicit decorator installation.
- Decorator eagerly types all nodes that have typed inputs
  (subsumes typing of constant cache, removing its typing
  side-channel and various spurious dependencies on the typer).
- Cut down typer interface to prevent inconsistently typed graphs.
- Remove verification from start, since it caused too much trouble
  with semi-wellformed nodes.
- Fix a couple of bugs on the way that got uncovered.

To do: verifying machine operators. Also, various conditions in the
verifier are currently commented out, because they don't yet hold.

BUG=
R=jarin@chromium.org,titzer@chromium.org

Review URL: https://codereview.chromium.org/658543002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24626 ce2b1a6d-e550-0410-aec6-3dcde31c8c00