Invalidate the instanceof cache in proxy-case of InstanceOfStub
after the changes in https://codereview.chromium.org/1492243003.

This didn't have a test case until https://codereview.chromium.org/1518773003

TEST=mjsunit/es6/instanceof-proxies.js
BUG=

Review URL: https://codereview.chromium.org/1519303002

Cr-Commit-Position: refs/heads/master@{#32821}