With the very same SMIs making up a big chunk
of the parser zone (especially for asm.js) it
makes sense to cache the AstValues for them.
This is not ideal yet, but already saves
hundreds (sic!) MBs of memory for Unity games.

Review-Url: https://codereview.chromium.org/2485423002
Cr-Commit-Position: refs/heads/master@{#40866}