This fixes the bogus {Word32Equal} comparison in the ToString builtin
implementing Object.prototype.toString to be a pointer-size {WordEqual}
comparison instead. Comparing just the lower half-word is insufficient
on 64-bit architectures.

R=jgruber@chromium.org
TEST=mjsunit/regress/regress-crbug-664506
BUG=chromium:664506

Review-Url: https://codereview.chromium.org/2496043003
Cr-Commit-Position: refs/heads/master@{#40963}