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=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/1210583002

Cr-Commit-Position: refs/heads/master@{#29280}