DiscIO: Add IsCached virtual function to BlobReader.

Source/Core/DiscIO/Blob.h

@@ -97,6 +97,9 @@ public:
     return false;
   }
 
+  // Returns true only for CachedBlobReader.
+  virtual bool IsCached() const { return false; }
+
 protected:
   BlobReader() {}
 };

Source/Core/DiscIO/CachedBlob.cpp

@@ -219,6 +219,8 @@ public:
     return m_cache_filler->Read(offset, size, out_ptr) || m_reader->Read(offset, size, out_ptr);
   }
 
+  bool IsCached() const override { return true; }
+
 private:
   // A shared object does the cache filling for sensible CopyReader behavior.
   const std::shared_ptr<CacheFiller> m_cache_filler;
@@ -228,11 +230,17 @@ private:
 
 std::unique_ptr<BlobReader> CreateCachedBlobReader(std::unique_ptr<BlobReader> reader)
 {
+  if (reader->IsCached())  // This is already CachedBlobReader.
+    return reader;
+
   return std::make_unique<CachedBlobReader>(std::move(reader), false);
 }
 
 std::unique_ptr<BlobReader> CreateScrubbingCachedBlobReader(std::unique_ptr<BlobReader> reader)
 {
+  if (reader->IsCached())  // This reader is already a CachedBlobReader.
+    return reader;
+
   return std::make_unique<CachedBlobReader>(std::move(reader), true);
 }