✨ MLIR - Support merging of more complex gates

[denialhaag]

Problem Statement

Currently, the merge-rotation-gates pass of MQTOpt can only merge gphase, rx, ry, rz, rxx, ryy, and rzz gates. The pass currently cannot merge xxminusyy, xxplusyy, u, and u2 gates.

Proposed Solution

It would be nice if the merge-rotation-gates could simplify consecutive applications of xxminusyy, xxplusyy, u, and u2.

One point of reference is the Qiskit pass for simplifying generic u gates.

For further discussion, see #1019.

[burgholzer]

I tagged #1407 to close this issue in an order to eventually reduce our number of open issues.
The new dialects already have quite a few more canonicalizations for the two-qubit rotation gates as well.
So I believe this may reasonably be closed after the Quaternion PR is merged.