The dynamic map checks operator loads the feedback vector and performs
map checks against each map and handler entry in the feedback against
the incoming map and handler.

Instead of emitting code to iterate over this feedback vector at
runtime, we unroll this loop at compile time.

The generated code is similar to this pseudocode:

  length = feedback_slot.length
  if length >= 4: goto labels[3]
  if length == 3: goto labels[2]
  if length == 2: goto labels[1]
  if length == 1: goto labels[0]

  labels[3]:
    map = load(feedback_slot, 6)
    if incoming_map == map goto handler_check(7)
    goto labels[2]
  labels[2]:
    map = load(feedback_slot, 4)
    if incoming_map == map goto handler_check(5)
    goto labels[1]
  labels[1]:
    map = load(feedback_slot, 2)
    if incoming_map == map goto handler_check(3)
    goto labels[0]
  labels[0]:
    map = load(feedback_slot, 0)
    if incoming_map == map goto handler_check(1)
    bailout

  handler_check (index):
    handler = load(feedback_slot, index)
    if incoming_handler ==  handler goto done
    deoptimize

Bug: v8:10582, v8:9684
Change-Id: I64d64ff8eda664e4d476bf1b2612e26a344e98a6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2339960
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69797}