Vector-based KeyedLoadIC MISS logic needs improvement.

Vector-based KeyedLoadIC MISS logic needs improvement.

Two issues:
1) We would trace the MISS two times on vector-based KeyedLoadIC
   with a string key in MEGAMORPHIC state.
2) There was a confusing asymmetry in the handling of going
   GENERIC. This change makes it the same whether
   --vector-ics is on or not.

BUG=

Review URL: https://codereview.chromium.org/839893005

Cr-Commit-Position: refs/heads/master@{#26105}