Optimize AOT's switchable calls for the monomorphic case.

runtime/vm/debugger_dbc.cc

Index: runtime/vm/debugger_dbc.cc
diff --git a/runtime/vm/debugger_dbc.cc b/runtime/vm/debugger_dbc.cc
index b4b6c1f059f85e3019f9aa94e08208906cef6e96..ac21678dafef9a8a56ec58c3a21792735917e82b 100644
--- a/runtime/vm/debugger_dbc.cc
+++ b/runtime/vm/debugger_dbc.cc
@@ -35,7 +35,7 @@ void CodeBreakpoint::PatchCode() {
   const Code& code = Code::Handle(code_);
   const Instructions& instrs = Instructions::Handle(code.instructions());
   {
-    WritableInstructionsScope writable(instrs.EntryPoint(), instrs.size());
+    WritableInstructionsScope writable(instrs.PayloadStart(), instrs.size());
     saved_value_ = *CallInstructionFromReturnAddress(pc_);
     switch (breakpoint_kind_) {
       case RawPcDescriptors::kIcCall:
@@ -80,7 +80,7 @@ void CodeBreakpoint::RestoreCode() {
   const Code& code = Code::Handle(code_);
   const Instructions& instrs = Instructions::Handle(code.instructions());
   {
-    WritableInstructionsScope writable(instrs.EntryPoint(), instrs.size());
+    WritableInstructionsScope writable(instrs.PayloadStart(), instrs.size());
     switch (breakpoint_kind_) {
       case RawPcDescriptors::kIcCall:
       case RawPcDescriptors::kUnoptStaticCall: