Since deferred return/async return/rethrow commands don't have an associated
statement, we can reuse their entries in the deferred command list.

Bug: v8:4280
Bug: v8:6218
Change-Id: Ic51fb735d4ea13ebea1d5e456ec5689c0d98a4ac
Reviewed-on: https://chromium-review.googlesource.com/506077
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45344}