Cleanup 1: split parts into their own libraries, remove unused imports

pkg/compiler/lib/src/ssa/codegen_helpers.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of ssa;
+import '../compiler.dart' show Compiler;
+import '../constants/values.dart';
+import '../elements/elements.dart';
+import '../js_backend/js_backend.dart';
+import '../types/types.dart';
+import '../universe/selector.dart' show Selector;
+
+import 'nodes.dart';
 
 /**
  * Replaces some instructions with specialized versions to make codegen easier.
  * Caches codegen information on nodes.
  */
 class SsaInstructionSelection extends HBaseVisitor {
   final Compiler compiler;
   HGraph graph;
 
   SsaInstructionSelection(this.compiler);
@@ skipping 743 matching lines @@
     }
 
     // If [thenInput] is defined in the first predecessor, then it is only used
     // by [phi] and can be generated at use site.
     if (identical(thenInput.block, end.predecessors[0])) {
       assert(thenInput.usedBy.length == 1);
       markAsGenerateAtUseSite(thenInput);
     }
   }
 }