Introduce a new Symbol comparison feedback bit in the lattice and
collect that feedback on Equal/StrictEqual in Ignition. Utilize this
feedback in TurboFan by adding a dedicated CheckSymbol operator to
check for symbol inputs. This way we can optimize Symbol comparison
where TurboFan doesn't know anything statically about either side, or
abstract equality comparisons where TurboFan doesn't statically know
anything about one side.

BUG=v8:6278,v8:6344,v8:6423
R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2893263002
Cr-Commit-Position: refs/heads/master@{#45455}