dart2js cps: Make register allocation prioritize phi elimination.

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

 // Expectation for test: 
 // // This test illustrates an opportunity to remove redundant code by
 // // propagating inforamtion after inlining.
 // //
 // // The code below inlines `foo` twice, but we don't propagate that we already
 // // know from the first `foo` that `a` is an int, so the second check can be
 // // removed entirely.
 // 
 // import 'package:expect/expect.dart';
 // 
 // main() {
 //   var a = nextNumber();
 //   action(foo(a));
 //   action(foo(a));
 // }
 // 
 // foo(x) {
 //   if (x is! int) throw "error 1";
 //   return x + 5 % 100;
 // }
 // 
 // @NoInline() @AssumeDynamic()
 // nextNumber() => int.parse('33');
 // 
 // @NoInline()
 // 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);
 }