Implement basic array prefetching hints in Hydrogen.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
===================================================================
--- src/hydrogen-instructions.h	(revision 13350)
+++ src/hydrogen-instructions.h	(working copy)
@@ -4367,11 +4367,12 @@
 
 
 class HLoadKeyed
-    : public HTemplateInstruction<3>, public ArrayInstructionInterface {
+    : public HTemplateInstruction<4>, public ArrayInstructionInterface {
  public:
   HLoadKeyed(HValue* obj,
              HValue* key,
              HValue* dependency,
+             HValue* prefetch_distance,
              ElementsKind elements_kind)
       : bit_field_(0) {
     bit_field_ = ElementsKindField::encode(elements_kind);
@@ -4379,6 +4380,7 @@
     SetOperandAt(0, obj);
     SetOperandAt(1, key);
     SetOperandAt(2, dependency != NULL ? dependency : obj);
+    SetOperandAt(3, prefetch_distance);
 
     if (!is_external()) {
       // I can detect the case between storing double (holey and fast) and
@@ -4424,6 +4426,10 @@
     return OperandAt(2);
   }
   bool HasDependency() const { return OperandAt(0) != OperandAt(2); }
+  HValue* prefetch_distance() { return OperandAt(3); }
+  void SetPrefetchDistance(HValue* prefetch_distance) {
+    SetOperandAt(3, prefetch_distance);
+  }
   uint32_t index_offset() { return IndexOffsetField::decode(bit_field_); }
   void SetIndexOffset(uint32_t index_offset) {
     bit_field_ = IndexOffsetField::update(bit_field_, index_offset);