dart2js cps: Make register allocation prioritize phi elimination.

tests/compiler/dart2js/cps_ir/expected/redundant_condition.js

Index: tests/compiler/dart2js/cps_ir/expected/redundant_condition.js
diff --git a/tests/compiler/dart2js/cps_ir/expected/redundant_condition.js b/tests/compiler/dart2js/cps_ir/expected/redundant_condition.js
index 1e043aaedb85a71a730c7c2f11043d92cb3579f8..a3a2ae0be88bea6046be82cb410847d5deb94042 100644
--- a/tests/compiler/dart2js/cps_ir/expected/redundant_condition.js
+++ b/tests/compiler/dart2js/cps_ir/expected/redundant_condition.js
@@ -26,10 +26,10 @@
 // action(v) => print(v);
 
 function() {
-  var a = V.nextNumber(), v0;
+  var a = V.nextNumber();
   if (!(typeof a === "number" && Math.floor(a) === a))
     throw H.wrapException("error 1");
-  v0 = a + 5;
-  V.action(v0);
-  V.action(v0);
+  a += 5;
+  V.action(a);
+  V.action(a);
 }