✨ Add `Barrier` Operation To MLIR Dialects

[MatthiasReumann]

Description

As of now, QASM's barrier operation can't be translated to the mqtref (and consequently mqtopt) dialect. Conceptually, Qiskit defines a barrier as

A directive for circuit compilation to separate pieces of a circuit so that any optimizations or re-writes are constrained to only act between barriers.

Implementation

TBD.

Some Ideas

• The most straight forward solution would be to add a BarrierOp and translate QASM's barrier to mqtref.barrier. A potential downside of this approach is that any optimization pattern must check for barriers.
• Alternatively, barriers might also be parsed into a custom basic sequence (as described in 🗺️ RFC: Quantum Dialect Revamp — Implementation Plan #1219) and optimizations run on these sequences (instead of mir::Module, let's say). This way patterns could ignore barriers altogether.