Use LoadInstanceType() rather than LoadMapInstanceType(), as this part of the
code is operating on a JS-accessible HeapObject rather than a Map.

BUG=chromium:633883, v8:5162
R=mstarzinger@chromium.org, bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/2207903002
Cr-Commit-Position: refs/heads/master@{#38297}