Make register allocator to disregard constant computation with no uses.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
diff --git a/runtime/vm/intermediate_language.h b/runtime/vm/intermediate_language.h
index 1cc6966ac24fd34ccc5f57acace07235d21ff98c..87b420b463125c73836650b2bb2f4674ed57f038 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -19,6 +19,7 @@ class BitVector;
 class BlockEntryInstr;
 class BufferFormatter;
 class ComparisonComp;
+class ConstantComp;
 class Computation;
 class ControlInstruction;
 class Definition;
@@ -863,6 +864,10 @@ class Definition : public Instruction {
   // Insert this definition after 'prev'.
   void InsertAfter(Instruction* prev);
 
+  // If this definition is a bind of a constant return it.
+  // Otherwise return NULL.
+  ConstantComp* AsConstant();
+
  private:
   intptr_t temp_index_;
   intptr_t ssa_temp_index_;