public CstProtoRef getCallSiteProto() {
    return callSiteProto;
  }
Gets the call site prototype. The call site prototype is provided as an argument to
invoke-polymorphic to enable type checking and type conversion.

@return {@code non-null;} Prototype reference for call site
android/android_framework/dex-maker/src/main/java/com/android/dx/rop/code/InvokePolymorphicInsn.java