Fix some bugs that blocked optimizations in the new IRs.

pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
index 4cc853c878f59dacfbea17a951d959d343450d67..f4e1617083d7f9727b2a9322be518ce7ad26d8f7 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
@@ -198,7 +198,7 @@ class Builder extends cps_ir.Visitor<Node> {
 
   List<Variable> translatePhiArguments(List<cps_ir.Reference> args) {
     return new List<Variable>.generate(args.length,
-         (int index) => getVariableReference(args[index]));
+         (int index) => getVariable(args[index].definition));

[asgerf, 2015/01/15 12:08:00]

getVariableReference() increases readCount.
getVariable() does not.

readCount is now set in addAssignment when we actually do the assignment.

This avoids spurious readCount in cases where the assignment is unnecessary
because register allocation merged the two variables.

[sigurdm, 2015/01/15 12:21:03]

Maybe put this explanation in a comment in the code.

[asgerf, 2015/01/15 13:05:37]

Done.

   }
 
   Statement buildContinuationAssignment(
@@ -247,6 +247,8 @@ class Builder extends cps_ir.Visitor<Node> {
 
     Statement first, current;
     void addAssignment(Variable dst, Variable src) {
+      ++src.readCount;
+      // `dst.writeCount` will be updated by the Assign constructor.
       if (first == null) {
         first = current = new Assign(dst, src, null);
       } else {