VM: Clean up and fix bugs in instructions patterns

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index bdfab97cd5745c957ab6ab8ed107a36b363f1301..8431674cfa11d66865b13632e1ee2b3ff979dfb0 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -3625,10 +3625,16 @@ class ObjectPool : public Object {
 
   static RawObjectPool* New(intptr_t len);
 
+  // Returns the pool index from the offset relative to a tagged RawObjectPool*,
+  // adjusting for the tag-bit.
   static intptr_t IndexFromOffset(intptr_t offset) {
     return (offset + kHeapObjectTag - data_offset()) / kBytesPerElement;
   }
 
+  static intptr_t OffsetFromIndex(intptr_t index) {
+    return element_offset(index) - kHeapObjectTag;
+  }
+
   void DebugPrint() const;
 
  private: