Separate out process_runs closure in Optimize1qGatesDecomposition

[mtreinish]

This commit splits out the internal closure process_runs that is internally used by the Optimize1qGatesDecomposition pass to process and synthesize a 1q run found in the circuit. This is just a mechanical change that is setting the stage for fixing the deadlock reported in issue #16591 that is being fixed by #16592. This change was originally part of #16592 but was split out to make the logic change made in #16592 more clear. This is just the mechanical pre-work needed for that, however while not strictly a fix this PR will need to backported to also backport #16592.

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description:
• I used the following tool to generate or modify code:

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[jakelishman]

I carved out the two hunks like so:

{ git cat-file blob pr/16595:crates/transpiler/src/passes/optimize_1q_gates_decomposition.rs | head -n460 | tail -n+378; } > new.txt

{ git cat-file blob pr/16595~:crates/transpiler/src/passes/optimize_1q_gates_decomposition.rs | head -n470 | tail -n+382; } > old.txt

then did diff -b old.txt new.txt and got

12c12
<                     match state.get_basis_set(PhysicalQubit::new(0), target, basis_gates.as_ref()) {
---
>             match state.get_basis_set(PhysicalQubit::new(0), target, basis_gates) {
19c19
<                     match state.get_basis_set(qubit, target, basis_gates.as_ref()) {
---
>             match state.get_basis_set(qubit, target, basis_gates) {
27,31c27
<                     state.get_euler_basis_set(
<                         PhysicalQubit::new(0),
<                         target,
<                         global_decomposers.as_ref(),
<                     )
---
>             state.get_euler_basis_set(PhysicalQubit::new(0), target, global_decomposers)
34,36c30,31
<                 state.target_basis_per_qubit[qubit.index()].get_or_init(|| {
<                     state.get_euler_basis_set(qubit, target, global_decomposers.as_ref())
<                 })
---
>         state.target_basis_per_qubit[qubit.index()]
>             .get_or_init(|| state.get_euler_basis_set(qubit, target, global_decomposers))
89d83
<         };

which is fairly easy to see is just rustfmt changes and a reference getting outlined.

[jakelishman]

Arguably this should be Option<&[String]> and make the corresponding bits Option::as_deref(), but I see get_euler_basis_set requires Option<&Vec<String>> for some reason, so let's leave it.

[mtreinish]

Heh, that's exactly what my process was while I was writing #16592 last night. I didn't want to make a deeper change as part of this to keep it targeted for backport so I just left it as &Vec.