port 3f6e0a4e (r40354)

  original commit message:
  RIP, handwritten KeyedLoadICStub, handwritten KeyedLoadIC_Megamorphic,
  and hydrogenized KeyedLoadGeneric!

BUG=

Review-Url: https://codereview.chromium.org/2430613003
Cr-Commit-Position: refs/heads/master@{#40388}