Optimize AOT's switchable calls for the monomorphic case.

runtime/vm/assembler.h

Index: runtime/vm/assembler.h
diff --git a/runtime/vm/assembler.h b/runtime/vm/assembler.h
index ceaadaa671ad78cd4a056956558385ea7ad3ab08..36279fbcc0ab9685bf079f2f06e7ae7be92ec70f 100644
--- a/runtime/vm/assembler.h
+++ b/runtime/vm/assembler.h
@@ -175,6 +175,8 @@ class AssemblerBuffer : public ValueObject {
   // Returns the position in the instruction stream.
   intptr_t GetPosition() const { return cursor_ - contents_; }
 
+  void Reset() { cursor_ = contents_; }
+
  private:
   // The limit is set to kMinimumGap bytes before the end of the data area.
   // This leaves enough space for the longest possible instruction and allows