This separation is needed to make two goals possible simultaneously:
* is should be possible to offer V8 components a simple, clean
  interface to TurboFan's low-level code generation that doesn't
  expose details about the TF.
* it should be possible to easily create new CodeAssembler "macros"
  that don't require a review from an OWNER of the compiler directory.

Review URL: https://codereview.chromium.org/1875583003

Cr-Commit-Position: refs/heads/master@{#35576}