What should we add?

In #10314 (comment) it was pointed out after #10314 merged that we can make some of the circuit instructions singletons pretty easily just as we did for parameterless gates. The Measure, Reset instruction are super straightforward to convert to singletons because they are single qubit/clbit constructs and do not take any parameters. Barrier also conceptually can be a "singleton" with a globally shared instance for barriers but as its variable width we'd need to store a distinct instance for each value of number of qubits that Barrier is instantiated with, which will require some custom handling as none of the singleton fixtures support this yet.