Fix some more shorten-64-to-32 warnings:

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
diff --git a/runtime/vm/flow_graph_optimizer.cc b/runtime/vm/flow_graph_optimizer.cc
index 7b31036f8f0fd16dd154d76f9ffa4d3d3ae7ab49..848cdb06c77ccb5c5c6c7d9f5a474a8da902f982 100644
--- a/runtime/vm/flow_graph_optimizer.cc
+++ b/runtime/vm/flow_graph_optimizer.cc
@@ -5822,10 +5822,11 @@ class Place : public ValueObject {
     return offset & ~(ElementSizeMultiplier(size) - 1);
   }
 
-  typedef BitField<Kind, 0, 3> KindBits;
-  typedef BitField<Representation, KindBits::kNextBit, 11> RepresentationBits;
-  typedef BitField<
-      ElementSize, RepresentationBits::kNextBit, 3> ElementSizeBits;
+  class KindBits : public BitField<uword, Kind, 0, 3> {};
+  class RepresentationBits :
+      public BitField<uword, Representation, KindBits::kNextBit, 11> {};
+  class ElementSizeBits :
+      public BitField<uword, ElementSize, RepresentationBits::kNextBit, 3> {};
 
   uword flags_;
   Definition* instance_;