dart2js: Redo known-type analysis after load-elimination
dart2js: Redo known-type analysis after load-elimination

Code like

  if (this.field is X) {
    (this.field as X)...
  }

after GVN and/or load-elimination becomes

  t1 = this.field;
  if (t1 is X) {
    (t1 as X)...
  }

so it is worth redoing the analysis that allows the elimination of
guaranteed-to-succeed 'as' check.  About 5% of the as-checks in a
large app are optimized out with this change.

R=sigmund@google.com

Review-Url: https://codereview.chromium.org/2657453004 .

dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk