Fix register hinting for phis.

runtime/vm/flow_graph_allocator.h

Index: runtime/vm/flow_graph_allocator.h
diff --git a/runtime/vm/flow_graph_allocator.h b/runtime/vm/flow_graph_allocator.h
index 51fe05f082b43ee4ed8d93fb298c2502c2e8e10a..dc894ccaae5b13560954f80a22419ae9fe84e44f 100644
--- a/runtime/vm/flow_graph_allocator.h
+++ b/runtime/vm/flow_graph_allocator.h
@@ -565,6 +565,7 @@ class LiveRange : public ZoneAllocated {
   UseInterval* first_use_interval() const { return first_use_interval_; }
   UseInterval* last_use_interval() const { return last_use_interval_; }
   Location assigned_location() const { return assigned_location_; }
+  Location* assigned_location_slot() { return &assigned_location_; }
   intptr_t Start() const { return first_use_interval()->start(); }
   intptr_t End() const { return last_use_interval()->end(); }