[MLIR] Add quantum.ctrl region op and ctrl-lowering pass

[maliasadi]

Context:
As part of adding Adjoint/Ctrl region/ops support to the graph-decomposition system, we need to reduce region-level modifiers to op-level nodes before the solver runs (Check the ADR for details). Controlled subcircuits need the same treatment: a Controlled(Op) produced by the solver (or a ctrl region coming from the frontend) must be reduced to op-level controlled gates before calling into the graph solver.

This PR adds the ctrl counterpart to the existing adjoint region infra via a quantum.ctrl region op and a ctrl-lowering pass.

Description of the Change:

We started simply by just implementing a forward walk pass over the region in the ctrl-lowering pass, for each op:

• gate: rebuilt with the region's controls appended to whatever controls it already carries.
• already-controlled gate: controls concatenate (via the control-merging method)
• nested quantum.ctrl: outer controls merged in and lowered on the next iteration
• structural ops (extract/insert/alloc/dealloc) and classical ops: just cloned

Related GitHub Issues:
[sc-125409]

[kipawaa]

Nice work @maliasadi 💯 Just requesting a couple of test cases that I think would be good to have

[paul0403]

Nice, thanks @maliasadi !

One thing I'm wondering is do we need a reference semantics version? Since the frontend qp.ctrl will be lowered to reference semantics.

I can help out with the qref.ctrl op (and the bidirectional semantics conversions) in another PR. For now, happy to get this one merged and unblock other graph decomp work!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.67%. Comparing base (a227f91) to head (fac801e).

Additional details and impacted files

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  Files         166      166           
  Lines       19171    19171           
  Branches     1822     1822           
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  Hits        18534    18534           
  Misses        475      475           
  Partials      162      162           

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[kipawaa]

Nice, thanks @maliasadi!