Previous to this patch, both the lithium and TurboFan register
allocators tracked allocated registers by "indices", rather than
the register codes used elsewhere in the runtime. This patch
ensures that codes are used everywhere, and in the process cleans
up a bunch of redundant code and adds more structure to how the
set of allocatable registers is defined.

Some highlights of changes:

* TurboFan's RegisterConfiguration class moved to V8's top level
  so that it can be shared with Crankshaft.
* Various "ToAllocationIndex" and related methods removed.
* Code that can be easily shared between Register classes on
  different platforms is now shared.
* The list of allocatable registers on each platform is declared
  as a list rather than implicitly via the register index <->
  code mapping.

Review URL: https://codereview.chromium.org/1287383003

Cr-Commit-Position: refs/heads/master@{#30913}