Start propagating non-primitive types in the backend, and fold instructions that know about the typ…

lib/compiler/implementation/ssa/types.dart

Index: lib/compiler/implementation/ssa/types.dart
===================================================================
--- lib/compiler/implementation/ssa/types.dart	(revision 6727)
+++ lib/compiler/implementation/ssa/types.dart	(working copy)
@@ -131,6 +131,9 @@
     HType desiredType = computeDesiredType(instruction);
     // If the desired type is conflicting just return the computed type.
     if (desiredType.isConflicting()) return newType;
+    // TODO(ngeoffray): Allow speculative optimizations on
+    // non-primitive types?
+    if (desiredType.isNonPrimitive()) return newType;
     return newType.combine(desiredType);
   }
 }