Adds unittests that test the fast register allocator correctly deals
with spills of Phi's between their definition and a predecessor block's
gap move to populate the Phi.

BUG=v8:9684

Change-Id: I17263058d5ac29088895ad3de7b3131315ec8fae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2299371
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69141}