Return DAGOpNode from dagcircuit.replace_block_with_op

[enavarro51]

Summary

Fixes #8724

Details and comments

This PR now returns the DAGOpNode that replaces the block from dagcircuit.replace_block_with_op.

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[coveralls]

Pull Request Test Coverage Report for Build 3244884583

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 151 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.2%) to 84.547%

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Files with Coverage Reduction	New Missed Lines	%
src/sabre_swap/sabre_dag.rs	1	94.44%
qiskit/extensions/quantum_initializer/squ.py	2	79.78%
src/sabre_swap/mod.rs	2	95.05%
src/results/marginalization.rs	40	65.52%
src/sampled_exp_val.rs	49	6.56%
src/optimize_1q_gates.rs	57	4.84%

Totals
Change from base Build 3242780185:	-0.2%
Covered Lines:	61752
Relevant Lines:	73039

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💛 - Coveralls

[kevinhartman]

Looks good to me. A nice improvement. Thanks!

[mtreinish]

Just tagging as on hold to prevent an accidental merge as we're waiting on CI for 0.22.0 which should hopefully go out tomorrow. After 0.22.0 is ready we can remove that label and tag automerge.