Fix assertion in ConstantPropagator::VisitInstanceOf.
Fix assertion in ConstantPropagator::VisitInstanceOf.

Propagated type doesn't have to match value representation because it is
affected by dominating checks (these checks will always deopt if there is
a mismatch making all subsequent code dead).

R=fschneider@google.com
BUG=http://dartbug.com/25362

Review URL: https://codereview.chromium.org/1567023002 .

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