Design doc:
https://docs.google.com/document/d/1n9ADWnDI-sw0OvdSmrthf61prmDqbDmQq-NSrQw2MVI/edit?usp=sharing

Most of this change follows directly what is discussed in the design
document. A few other things are also changed:

- PopulateReferenceMapsPhase is moved after ResolveControlFlowPhase so
  that it can make use of the decision regarding whether a value is
  spilled at its definition or later.
- SpillSlotLocator is removed. It was already somewhat confusing,
  because the responsibility for marking blocks as needing frames was
  split: in some cases they were marked by SpillSlotLocator, and in
  other cases they were marked by CommitSpillsInDeferredBlocks. With
  this change, that split responsibility would become yet more
  confusing if we kept SpillSlotLocator for the values that are spilled
  at their definition, so I propose a simpler rule that whatever code
  adds the spill move also marks the block.
- A few class definitions (LiveRangeBound, FindResult,
  LiveRangeBoundArray, and LiveRangeFinder) are moved without
  modification from register-allocator.cc to register-allocator.h so
  that we can refer to them from another cc file.

Bug: v8:10606
Change-Id: I374a3219a5de477a53bc48117e230287eae89e72
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2285390
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69345}