It is not always safe to allocate new heap objects in the JSGraph. We
might have to revisit this later once we do the canonicalization for
HeapConstants.

R=jarin@chromium.org
BUG=chromium:545364
LOG=n

Review URL: https://codereview.chromium.org/1413373002

Cr-Commit-Position: refs/heads/master@{#31421}