Follow-on to hydrogen fix keyed loads with string keys for vector ics.

r25817 optimized hydrogen keyed loads with string keys.
(https://codereview.chromium.org/755513003).
This CL adapts that work for the vector-ic keyed load.

BUG=

Review URL: https://codereview.chromium.org/820673002

Cr-Commit-Position: refs/heads/master@{#25903}