PPC: Vector ICs: Like megamorphic keyed koads, use a dummy vector for stores

PPC: Vector ICs: Like megamorphic keyed koads, use a dummy vector for stores

Port 9e7af9efc5857b3c7e23a77d257f3dfbea597753

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.

R=mvstanton@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1212493002

Cr-Commit-Position: refs/heads/master@{#29302}