Optimize AOT's switchable calls for the monomorphic case.

runtime/lib/stacktrace.cc

Index: runtime/lib/stacktrace.cc
diff --git a/runtime/lib/stacktrace.cc b/runtime/lib/stacktrace.cc
index a63d66dd805afdc02fbf18e3d71d3a6fdfaf569f..12d60585072ee620bd9dfc9d0858da594e4cf1c7 100644
--- a/runtime/lib/stacktrace.cc
+++ b/runtime/lib/stacktrace.cc
@@ -25,7 +25,7 @@ static void IterateFrames(const GrowableObjectArray& code_list,
         skip_frames--;
       } else {
         code = frame->LookupDartCode();
-        offset = Smi::New(frame->pc() - code.EntryPoint());
+        offset = Smi::New(frame->pc() - code.PayloadStart());
         code_list.Add(code);
         pc_offset_list.Add(offset);
       }