This change aims to introduce the separation of the RegisterAllocator model,
using the initial prototype for RegisterAllocatorGreedy as proof of concept.

Summary:
- new flag, turbo-greedy-regalloc, enabling the new allocator. Default
  false.
- separated RegisterAllocator into a base type and two derived,
  RegisterAllocatorLinear (the one currently used in TurboFan) and
  RegisterAllocatorGreedy (the new one).
- initial, untested impementation for the greedy allocator.

BUG=

Review URL: https://codereview.chromium.org/1061923005

Cr-Commit-Position: refs/heads/master@{#28015}