Keep track of loop nesting depth on FunctionState and use that to decide
whether to mark var as assigned. That also fixes the weird cornercase where a
loop body can have multiple expressions due to multiple declarations with
independent initializers in a single var-statement.

Change-Id: Ia24affde29e22e9464448fd390062f6dd983faf2
Reviewed-on: https://chromium-review.googlesource.com/c/1405037Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58707}