Improve MCMT synthesis for XGate base gate

[patelvyom]

This PR optimises the synthesis of multi-controlled multi-target (MCMT) gates when the base_gate is XGate(). The current implementation uses a generic V-chain construction, which is suboptimal for this case. We replace it with a more efficient decomposition:

• For n_target = 1, it reduces to a standard MCX gate.
• For n_target > 1, it uses a single MCX gate and 2*(t-1) CX gates, with at least one dirty ancilla available.
• This enables the use of optimized MCX decompositions (e.g., Update synth_mcx_kg24 to use relative phase Toffolis #14394).

A comparison plot is included below, showing that the proposed method achieves savings of hundreds to thousands of CX gates, depending on the circuit size. Here, MCMTX is the proposed decomposition.

Tagging @ShellyGarion and @alexanderivrii based on past discussions around MCX gates.

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[coveralls]

Pull Request Test Coverage Report for Build 16501202274

Details

• 26 of 27 (96.3%) changed or added relevant lines in 3 files are covered.
• 15 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.009%) to 87.76%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/transpiler/passes/synthesis/hls_plugins.py	10	11	90.91%

Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/symbol_expr.rs	1	73.94%
crates/qasm2/src/expr.rs	1	93.63%
crates/qasm2/src/parse.rs	6	97.56%
crates/qasm2/src/lex.rs	7	92.01%

Totals
Change from base Build 16481559458:	0.009%
Covered Lines:	81520
Relevant Lines:	92890

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💛 - Coveralls

[ShellyGarion]

Thank you @patelvyom for another high-quality and important contribution to qiskit!
I wonder if there is some specific use-case of MCMT-X gate?
I also have a few comments on the docs and the tests. Release notes are also needed.

[patelvyom]

Re: Uses of MCMTX gate:

• The primary motivation is in its use in a controlled version of classical-quantum adder (I plan to add a PR soon).
• I will add a PR for new fixed adder gate (that has Clifford + T decomposition). There exists an efficient controlled version of this adder that uses the MCMTX gate. This should improve gate counts in Improve Clifford+T synthesis for MultiplierGate #14682.

[ShellyGarion]

LGTM. I think that there are only some lint and indentation errors to fix.
Thanks!

[ShellyGarion]

I have a few minor comments on the docs. Other than that, I think it's ready to go :)

[ShellyGarion]

@patelvyom - thanks for another nice contribution to qiskit! I hope that it will also be useful for other arithmetic circuits.

@Cryoris - do you have any further comments?