Introducing the KeyAccumulator accidentally removed some crucial fast-paths.
This CL starts rewriting the KeyAccumulator, step-by-step introducing the
special cases again.

BUG=chromium:545503, v8:4758
LOG=y

Review URL: https://codereview.chromium.org/1707743002

Cr-Commit-Position: refs/heads/master@{#34532}