In the special case of KeyedLoadIC, where the key that is passed in is a
Name that is always the same we only checked for identity in both the
stub and the TurboFan case, which works fine for symbols and internalized
strings, but doesn't really work with non-internalized strings, where
the identity check will fail, the runtime will internalize the string,
and the IC will then see the original internalized string again and not
progress in the feedback lattice. This leads to tricky deoptimization
loops in TurboFan and constantly missing ICs.

This adds fixes the stub to always try to internalize strings first
when the identity check fails and then doing the check again. If the
name is not found in the string table we miss, since in that case the
string cannot match the previously recorded feedback name (which is
always a unique name).

In TurboFan we represent this checks with new CheckEqualsSymbol and
CheckEqualsInternalizedString operators, which validate the previously
recorded feedback, and the CheckEqualsInternalizedString operator does
the attempt to internalize the input.

Bug: v8:6936, v8:6948, v8:6969
Change-Id: I3f3b4a587c67f00f7c4b60d239eb98a9626fe04a
Reviewed-on: https://chromium-review.googlesource.com/730224Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48784}