This avoids needing to allocate a zonelist on the fly later, sorting variables_, for which we also need to keep track of order in the hashmap.

In a later phase we can make sure that Variable is always uniquely in either of params_, temps_ and ordered_variables_. In that case we can use a linked list through Variable.

BUG=v8:5209

Review-Url: https://codereview.chromium.org/2264053003
Cr-Commit-Position: refs/heads/master@{#38810}