This unifies the code between parser and preparser, and removes more code from
the pattern rewriter.

This makes "var x" without assignment and initializer in a loop pessimistically
marked as assigned, but that seems pretty unlikely since the variable will just
always be undefined. It is also still strictly better than what we had until
very recently since any var outside of the function scope used to be marked as
assigned. Now we only mark such variables as assigned.

Change-Id: Icb37ab249b2a79c2d57a5769bdb964b435cebf62
Reviewed-on: https://chromium-review.googlesource.com/c/1405228Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58736}