The old hydrogen stub (and runtime fallback) are still needed while they are
used in regexp.js.  These will go away once affected code has been ported.

Drive-by-fix: Fixed typo in GetFixedA{a,}rrayAllocationSize.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2384473002
Cr-Commit-Position: refs/heads/master@{#39952}