For deferred commands (such as in try-finally), some deferred commands
save and restore the accumulator using a result register (e.g. return,
throw, rethrow), while others don't (e.g. break, continue,
fall-through).

However, conditionally reading this result register that may not ever be
written caused it to be considered live from the start of the function,
as far as the liveness analysis could statically tell.

Now, we write the result register for all deferred commands, including
the fall-through. As a micro-optimization, we re-use the Smi command
tokeen to clobber the result, rather than emitting an LdaUndefined.

Bug: chromium:758472
Change-Id: I2ea65e2249b40ee6403216e654a8bb88d50bec3b
Reviewed-on: https://chromium-review.googlesource.com/635592
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47612}