[turbofan] Remove usage of Unique<T> from graph.

[turbofan] Remove usage of Unique<T> from graph.

The usage of Unique<T> throughout the TurboFan IR does not have any
advantage. There is no single point in time when they are initialized
and most use-sites looked through to the underlying Handle<T> anyways.
Also there already was a mixture of Handle<T> versus Unique<T> in the
graph and this unifies the situation to use Handle<T> everywhere.

R=bmeurer@chromium.org,titzer@chromium.org

Review URL: https://codereview.chromium.org/1314473007

Cr-Commit-Position: refs/heads/master@{#30458}