Revert of [ic] [stubs] Don't use Code::flags in megamorphic stub cache hash computations. (patchset #2 id:20001 of https://codereview.chromium.org/2147433002/ )

Reason for revert:
Unfortunately, the performance issues are still there on Android.

Original issue's description:
> [ic] [stubs] Don't use Code::flags in megamorphic stub cache hash computations.
>
> This should avoid weird performance issues when changing layout of Code::flags field.
>
> BUG=chromium:618701
>
> Committed: https://crrev.com/aa4140b1ccc114b82700471513c715f68b7c5dac
> Cr-Commit-Position: refs/heads/master@{#37755}

TBR=jkummerow@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=chromium:618701

Review-Url: https://codereview.chromium.org/2147213004
Cr-Commit-Position: refs/heads/master@{#37801}