port 9e7af9ef (r29280).

original commit message:

    It's useful for the megamorphic keyed store case to not require a
    vector and slot as input. Analogous to the load case, we have a dummy
    one-ic-slot vector to aid. Since the only kind of MISS is for
    megamorphic cache stub failures, we don't need the real vector.
    The reason is that megamorphic cache stub failures don't result in any
    change to the type feedback vector state.

BUG=

Review URL: https://codereview.chromium.org/1212653011

Cr-Commit-Position: refs/heads/master@{#29410}