When --vector-ics is true, we still tail-call to the hand-written
megamorphic KeyedLoadIC (formerly "generic"). Now that this code uses
the megamorphic cache, it needs to deal properly with the vector and
slot registers. Achieve this with a sentinel vectors/slot combo.

R=dcarney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/892173002

Cr-Commit-Position: refs/heads/master@{#26381}