port 16843e23 (r26381)

original commit message:

  Megamorphic KeyedLoadIC needs special handling for vector ics.

  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.

BUG=
R=weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/910623002

Cr-Commit-Position: refs/heads/master@{#26508}