Cleanup after the interceptor refactoring work, and add a new SSA instruction for an interceptor, s…

sdk/lib/_internal/compiler/implementation/ssa/tracer.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/tracer.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/tracer.dart	(revision 15192)
+++ sdk/lib/_internal/compiler/implementation/ssa/tracer.dart	(working copy)
@@ -314,6 +314,11 @@
     return "Integer check: $value";
   }
 
+  String visitInterceptor(HInterceptor node) {
+    String value = temporaryId(node.inputs[0]);
+    return "Intercept: $receiver";
+  }
+
   String visitInvokeClosure(HInvokeClosure node)
       => visitInvokeDynamic(node, "closure");