Adds support for register allocation within a block to the fast
register allocator. Also adds some unittests covering basic
register allocation. No support yet for spill slot allocation,
so functions that spill don't work yet.

BUG=v8:9684

Change-Id: I91d0fc0660d7b65f59235242fd5e3b1a7618d813
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2297467
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69045}