When there are multiple entries into a deferred block region, ensure
that we freeze the set of deferred spill virtual registers when we have
processed the first entry point to that deferred block. This ensures
that we don't add another vreg into the set of deferred spills, and
then specify that that deferred spill slot is live across the whole
deferred block, when it is only live from certain entry points.

BUG=chromium:1227568,v8:9684

Change-Id: I647851be9a00fba262768e4f1a7846669b585a2e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3021178Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75704}