Reapply "Refactor constant part." (r4958) with fixes.

frog/leg/ssa/tracer.dart

Index: frog/leg/ssa/tracer.dart
diff --git a/frog/leg/ssa/tracer.dart b/frog/leg/ssa/tracer.dart
index 445dda7780d7473310191bf7d7461e60f8aba35f..a012725448a32d9163cb4e06e991297f62e116df 100644
--- a/frog/leg/ssa/tracer.dart
+++ b/frog/leg/ssa/tracer.dart
@@ -217,6 +217,8 @@ class HInstructionStringifier implements HVisitor<String> {
     return "Break: (B${target.id})";
   }
 
+  String visitConstant(HConstant constant) => "Constant ${constant.constant}";
+
   String visitDivide(HDivide node) => visitInvokeStatic(node);
 
   String visitEquals(HEquals node) => visitInvokeStatic(node);
@@ -327,8 +329,6 @@ class HInstructionStringifier implements HVisitor<String> {
   String visitLess(HLess node) => visitInvokeStatic(node);
   String visitLessEqual(HLessEqual node) => visitInvokeStatic(node);
 
-  String visitLiteral(HLiteral literal) => "Literal ${literal.value}";
-
   String visitLiteralList(HLiteralList node) {
     StringBuffer elementsString = new StringBuffer();
     for (int i = 0; i < node.inputs.length; i++) {