- Use representation information provided by the type system to skip SMI
  checks.
- Fix combining of ChangeTaggedToFloat64 with JSToNumber now that JS
  operators can produce control.
- Remove the unnecessary abstraction of smi/field offsets.
- Improve unit test coverage.
- Various cosmetic fixes.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/1018873002

Cr-Commit-Position: refs/heads/master@{#27250}