ExpressionClassifier was used just for transmitting information back and forth
to DeclareFormalParameters.

As a bonus, we now do the Scope::IsDeclaredParameter check only when we're going
to use the information it produces.

BUG=v8:6092,v8:6474

Change-Id: Ib5ac6a779705caa74e933e1c6f03eaaf0f49bf05
Reviewed-on: https://chromium-review.googlesource.com/455836
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45809}