LoadInternalField performs some redundant checks in the context of
fast accessors.
This improves the speedup from 20% to 60% for next/previousSibling in
the Dromaeo DOM traversal benchmarks.

BUG=chromium:508898

Review-Url: https://codereview.chromium.org/2186593002
Cr-Commit-Position: refs/heads/master@{#38535}