For vector-based keyed store ics, we need to know the current
KeyedAccessStore mode on ic MISS, and to produce optimized code.

We can't store this mode, which can change on any MISS in the IC
without patching. Therefore, this CL makes sure that the information is
redundantly available in the handlers embedded in the IC. This way,
when --vector-stores is turned on, we'll be able to extract that
information from the vector which maintains a list of these handlers.

BUG=

Review URL: https://codereview.chromium.org/1312693004

Cr-Commit-Position: refs/heads/master@{#30378}