VectorICs: Create a StoreICState to more easily create matching code stubs.

VectorICs: Create a StoreICState to more easily create matching code stubs.

This follows the logic of the load ics, in that the base extra ic state is
better encapsulated.

Introduce flag vector_stores to aid development of vector-based store ics.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/1151253004

Cr-Commit-Position: refs/heads/master@{#28576}