LibJS: Don't crash when attempting to load from an invalid reference

Previously, attempting to load a value from an invalid reference would
cause a crash. We now return a CodeGenerationError rather than hitting
an assertion. This is not a complete solution, as ideally we would want
to return a ReferenceError, but this now matches the behavior we see
when we attempt to store something to an invalid reference.

Userland/Libraries/LibJS/Bytecode/Generator.cpp

@@ -268,7 +268,10 @@ CodeGenerationErrorOr<Optional<Generator::ReferenceRegisters>> Generator::emit_l
         }
         return Optional<ReferenceRegisters> {};
     }
-    VERIFY_NOT_REACHED();
+    return CodeGenerationError {
+        &node,
+        "Unimplemented/invalid node used a reference"sv
+    };
 }
 
 CodeGenerationErrorOr<void> Generator::emit_store_to_reference(JS::ASTNode const& node)