Make sure that the testAddFn is always optimized.

Bug: v8:8220
Change-Id: I38543dc4cf99df6a379c9474ce275af2c9ef015d
Reviewed-on: https://chromium-review.googlesource.com/c/1355401
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57959}