Rename InstructionsReader to ImageReader for symmetry with ImageWriter.

runtime/vm/clustered_snapshot.h

Index: runtime/vm/clustered_snapshot.h
diff --git a/runtime/vm/clustered_snapshot.h b/runtime/vm/clustered_snapshot.h
index 9f707b689bd74a1c744045b891ff919e39ec7611..2a1121c9c8c3feabc7690e6e220eb144ac8d90ec 100644
--- a/runtime/vm/clustered_snapshot.h
+++ b/runtime/vm/clustered_snapshot.h
@@ -336,11 +336,11 @@ class Deserializer : public StackResource {
   }
 
   RawInstructions* GetInstructionsAt(int32_t offset) {
-    return instructions_reader_->GetInstructionsAt(offset);
+    return image_reader_->GetInstructionsAt(offset);
   }
 
   RawObject* GetObjectAt(int32_t offset) {
-    return instructions_reader_->GetObjectAt(offset);
+    return image_reader_->GetObjectAt(offset);
   }
 
   RawApiError* VerifyVersionAndFeatures(Isolate* isolate);
@@ -359,7 +359,7 @@ class Deserializer : public StackResource {
   Zone* zone_;
   Snapshot::Kind kind_;
   ReadStream stream_;
-  InstructionsReader* instructions_reader_;
+  ImageReader* image_reader_;
   intptr_t num_base_objects_;
   intptr_t num_objects_;
   intptr_t num_clusters_;