llvm_ir_generator: don't mark non-constant attribute loads as invariant.

Oops.

artiq/compiler/transforms/llvm_ir_generator.py

@@ -811,6 +811,7 @@ class LLVMIRGenerator:
                 llptr = self.llbuilder.gep(obj, [self.llindex(0), self.llindex(index)],
                                            inbounds=True, name="ptr.{}".format(insn.name))
                 llval = self.llbuilder.load(llptr, name="val.{}".format(insn.name))
+                if types.is_instance(typ) and attr not in typ.constant_attributes:
                     llval.metadata['invariant.load'] = self.empty_metadata
                 return llval