Fix LoadOptimizer's handling of load/stores with constant indices for TypedData.

runtime/vm/bitfield.h

Index: runtime/vm/bitfield.h
diff --git a/runtime/vm/bitfield.h b/runtime/vm/bitfield.h
index 4ef7766b5acf50a359451251ee13f51dca92a159..6108065f1e6ace1164605d66dd32837568a6a0b4 100644
--- a/runtime/vm/bitfield.h
+++ b/runtime/vm/bitfield.h
@@ -14,6 +14,8 @@ static const uword kUwordOne = 1U;
 template<typename T, int position, int size>
 class BitField {
  public:
+  static const intptr_t kNextBit = position + size;
+
   // Tells whether the provided value fits into the bit field.
   static bool is_valid(T value) {
     return (static_cast<uword>(value) & ~((kUwordOne << size) - 1)) == 0;