Change from as operator to direct cast or the opposite

This:

``` csharp
var nullLiteral = returnElse.Expression as LiteralExpressionSyntax;
```

Becomes:

``` csharp
var nullLiteral = (LiteralExpressionSyntax)returnElse.Expression;
```

Or the opposite, turn a direct cast into an expression with the `as` operator.

Severity is `Hidden`.
Diagnostic id is `CC0028`.
Category: `Refactoring`
