Runtime_OptimizeFunctionOnNextCall and Runtime_DeoptimizeFunction asserts that
the argument is a JSFunction object.These are used by fuzzers to get coverage
of optimizations in compiler. Having an assert causes a fuzzer test to fail
when OptimizeFunctionOnNextCall is called on objects that are not functions.
We can instead, silently return on such calls.

BUG=chromium:601391
LOG=N

Review URL: https://codereview.chromium.org/1883603002

Cr-Commit-Position: refs/heads/master@{#35539}