Monomorphic loads are quite common and it is important to keep these
load accesses fast. Dynamic map checks increases the overhead for these
monomorphic accesses by having to actually verify the IC state and check
against a map from the feedback vector This was causing a significant
(~2-3%) regression in JavaScript duration. To keep the common case of
monomorphic checks fast, we now want to add a check against expected
map (which passes in most cases) and move the rest of the checks to a
builtin. i.e. we want dynamic map checks (when generating the code for
loads in monomorphic state) to look like:

if (incoming_map != HeapConstant(expected_map))
  call_builtin;

This helps us to keep the most common case fast and still gets the
benefits of dynamic map checks.

This cl is the first in the series of cls that will add this
functionality. This cl makes the expected_map available for dynamic map
checks operator. In follow up cls, we will add a builtin and update
the code to use the builtin.


Bug: v8:10582
Change-Id: I10992c6ba1fb005592de962310c208cff6829119
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2397894Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69798}