[TurboProp] Add cross-block allocation support to fast reg alloc

Adds support for tracking register allocations across basic block
boundaries to the fast register allocator. For now we still spill
on loop headers, and spill when merging register states if the
register state isn't exactly the same.

BUG=v8:9684

Change-Id: I2aaf992fe8b0a5c698b1e44526951c63aedbe86c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2300480
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69181}