Besides avoiding the weird hack of inserting a statement at the 0th
index of the function body, we also avoid allocating (and initializing)
the variable if it's unreferenced (which I'd wager is the common case).

Bug: v8:6092
Change-Id: If917d422bb4818cf21e8272aa786ca84d4472802
Reviewed-on: https://chromium-review.googlesource.com/784092Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49646}