[runtime] Speed up allocating instances in the runtime by having a quick-check for inobject slack tracking.

This speeds up
https://github.com/kpdecker/six-speed/blob/master/tests/object-assign/object-assign.es5
by over 5%.

Review URL: https://codereview.chromium.org/1690923002

Cr-Commit-Position: refs/heads/master@{#33917}