When a slow mode for-in loop is compiled with Crankshaft we
unconditionally deoptimize when we hit an object with a usable
enum-cache (which is currently hidden by another CL), and obviously
we don't learn anything from that.

R=jarin@chromium.org
BUG=v8:3650
LOG=n

Review URL: https://codereview.chromium.org/1611933003

Cr-Commit-Position: refs/heads/master@{#33427}