Only expose top-level functions for DefineOutputs and AllocateRegisters in
the mid-tier register allocator, rather than exposing the MidTierRegisterAllocator
object, to be in-line with AllocateSpillSlots and PopulateReferenceMaps.

BUG=v8:9684

Change-Id: I93dcff77f5e50dab9b373b4415029361078d58e1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2323361
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69226}