Adds support for Phis to be allocated to the fast register
allocator. Registers used for Phis are marked specially between
the point where the Phi is defined, and the gap-move's in the
predecessor blocks which populate the Phi value, since if the
Phi is spilled then all predecessor blocks must also spill the
Phi even if they were already allocated.

BUG=v8:9684

Change-Id: Iebe90495b83df655d3335a7d55874123f3b27f8d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2299366
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69139}