There's no point in having %_IsFunction as inline intrinsic, as it
is only used in non performance critical code, which is already full
of runtime calls anyway, so %IsFunction will do the trick as well.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1658123002

Cr-Commit-Position: refs/heads/master@{#33660}