Flow analysis: Simplify ExpressionInfo class hierarchy.

After landing https://dart-review.googlesource.com/c/sdk/+/304211, I
did some testing using `pkg/front_end/tool/benchmarker.dart`, and
found that it regressed performance of the CFE more than I was
comfortable with:

instructions:u: 0.2097% +/- 0.0004%
branches:u: 0.2310% +/- 0.0006%

The culprit seemed to be the complex class hieararchy of
ExpressionInfo. This change simplifies down to just 4 classes:

- ExpressionInfo (the root),

- _Reference (for references to variables, properties, `this`, and
  `super`),

- TrivialVariableReference (for references to variables that don't
  contain a boolean value that influences the flow model), and

- _NullInfo (for `null` literals).

Notably, the simplified class hierarchy doesn't contain any diamond
shapes (which should simplify `is` testing), and the only overrides
are `toString` and `isNull`, so many methods should now be possible to
inline.

This change results in the following performance improvement:

instructions:u: -0.2768% +/- 0.0003%
branches:u: -0.2950% +/- 0.0004%

Which more than offsets the previous regression.

Change-Id: I02f55a621ca6601ddc1d59bf79e386c1703e09bd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/306907
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>