Previously, accumulators and registers each had a single element cache,
distinct from the local register cache. This meant that

 a) Dead accumulator state nodes were not re-used if the accumulator
    became live.
 b) Functions with only one parameter (the this object) or only one
    local register could not reuse the single-valued state value node
    of the accumulator.

This patch introduces heavier re-use of state-value nodes, decreasing
memory use when building the graph and decreasing the number of nodes
created overall.

Change-Id: Ie3cc6913483aab0819d99be382eb2cb42de8c3d2
Reviewed-on: https://chromium-review.googlesource.com/440926Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43148}