Make register allocator to disregard constant computation with no uses.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 18bd1b311f2d2025fa4e9f9b7fe84e670acd0cdd..e82d2368b9f36e410b72f057d0d3cfc0a5fd92c8 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -219,6 +219,12 @@ void Definition::InsertAfter(Instruction* prev) {
 }
 
 
+ConstantComp* Definition::AsConstant() {
+  BindInstr* bind = AsBind();
+  return (bind != NULL) ? bind->computation()->AsConstant() : NULL;
+}
+
+
 void ForwardInstructionIterator::RemoveCurrentFromGraph() {
   current_ = current_->RemoveFromGraph(true);  // Set current_ to previous.
 }