IsFixedArray is too broad in many cases and should be replaced by
IsFixedArrayExact in the objects stats collection.

Bug: v8:7266
Change-Id: I3d5de8b70dc596a391ffdc2a5b4bdeaa5d437712
Reviewed-on: https://chromium-review.googlesource.com/948502Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51739}