Qiskit / qiskit / 7033895831

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Decomposition of single R gate in RR basis should be one R gate, not two (#11304)

* Decomposition of single R gate in RR basis should be one R gate, not two

Currently, a single R gate may come out of RR-decomposition as two R gates, which is obviously not ideal.

For example, `RGate(0.1, 0.2)` is currently decomposed (in RR basis) as

```
   ┌────────────────┐┌──────────┐
0: ┤ R(-3.0416,0.2) ├┤ R(π,0.2) ├
   └────────────────┘└──────────┘
```

Two `R(𝜗, 𝜑)` gates with the same 𝜑 parameter can be combined into one by simply adding up the 𝜗 values (giving us `R(0.1, 0.2)`, unsurprisingly).

In terms of `U(𝜗, 𝜑, 𝜆)`, it is the case when `𝜑 = -𝜆` that the two R-gates we construct have the same second parameter and therefore should be expressed as a single R gate.
For example, `U3Gate(0.1, 0.2, -0.2)` currently produces this RR-decomposition:

```
   ┌───────────────────┐┌─────────────┐
0: ┤ R(-3.0416,1.7708) ├┤ R(π,1.7708) ├
   └───────────────────┘└─────────────┘
```

which also unnecessarily consists of two R gates instead of just one.

This commit adds the two examples above as unit tests, ensuring they RR-decompose two just one R gate, as well as the code changes to make these two new tests pass, along with all existing tests, of course.

The condition for this special case is that the 𝜑 parameters of the two R-gates we would emit are the same (thus `mod_2pi(PI / 2. - lam)=mod_2pi(0.5 * (phi - lam + PI)`, simplified as `mod_2pi((phi + lam) / 2)=0`).

* Add release note

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