Vars without initialisers don't need to allocate a VariableProxy, as the
proxy expression is not really needed for anything. So, we can special
case declaration parsing to look ahead for a '=' (plus a few other
cases), and skip the variable proxy allocation if it isn't there.

As a side-effect, variables that are only declared but never used are
no longer marked is_used, and thus not allocated. This saves on
generating dead code.

Change-Id: Ie4f04c6b5c1138df4c2e17acf1f0150459b3b571
Reviewed-on: https://chromium-review.googlesource.com/c/1434376
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59129}