Correctly updating global phase when removing gates that are identity up to a global phase (backport #13785)

[mergify]

Summary

This fixes a bug in the RemoveIdentityEquivalent transpiler pass (the pass was introduced in #12384), where unitary gates close to identity up to a global phase were removed from the circuit but the global phase of the circuit was not updated. This includes, for instance, single-qubit unitary gates with matrices close to [[ $e^{i\theta}$, 0], [0, $e^{i\theta}$]] for fix floating-point values of $\theta$, standard gates $R_X(2\pi)$, $R_Y(2\pi)$, $R_Z(2\pi)$ (whose matrices are close to $-I$), and some others. Now the pass removes these gates and correctly updates the global phase of the circuit. In particular, the pass now handles non-parameterized GlobalPhaseGates as well.

Fixes #13778.

Details and comments:

The pass ignores parameterized gates. We could probably relatively easily extend it to handle parameterized global phase gates.

The thing to notice in the current documentation of the pass is that the current condition for removing a gate is indeed satisfied for gates of the form $e^{i\theta} I$. And in fact the opposite is also true (thanks to @ShellyGarion for helping me work out the math): if the removal condition is satisfied, then a gate is necessarily of the form $e^{i\theta} I$.

Lol, yet another change after the #13759 was merged is to now also explicitly handle Unitary gates.

Thanks to @Cryoris and @ShellyGarion for joint debugging of the problem.

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This is an automatic backport of pull request #13785 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	modified:   qiskit/transpiler/passes/optimization/remove_identity_equiv.py
	new file:   releasenotes/notes/fix-phase-in-remove-id-equivalent-6480da0c62f20df1.yaml
	modified:   test/python/transpiler/test_remove_identity_equivalent.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   crates/accelerate/src/remove_identity_equiv.rs

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[coveralls]

Pull Request Test Coverage Report for Build 13262165759

Details

• 21 of 21 (100.0%) changed or added relevant lines in 1 file are covered.
• 16 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.002%) to 88.92%

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Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/remove_identity_equiv.rs	3	93.55%
crates/circuit/src/operations.rs	3	89.57%
crates/qasm2/src/lex.rs	4	91.73%
crates/qasm2/src/parse.rs	6	97.62%

Totals
Change from base Build 13187458716:	0.002%
Covered Lines:	79166
Relevant Lines:	89031

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💛 - Coveralls