Avoid deepcopy in sabre_swap (#5316)

This commit changes the deepcopy usage in the sabreswap pass to a
shallow copy. In sabre_swap the deepcopy was run on the DAGCircuit node
when remapping it, however this was not necessary because it's not
actually used where shared references matter. The output from the
remapper is not used directly and instead a copy of the DAGNode object
is just used as a container for the required args in
apply_operation_back() where a new DAGNode is always created from the
op, qargs, etc. The remapping just replaces the qargs parameter with the
remapped one. So this commit changes the deepcopy to a shallow copy
which won't have the performance overhead.

Fixes #5197

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/transpiler/passes/routing/sabre_swap.py

@@ -13,7 +13,7 @@
 """Routing via SWAP insertion using the SABRE method from Li et al."""
 
 import logging
-from copy import deepcopy
+from copy import copy
 from itertools import cycle
 import numpy as np
 
@@ -336,7 +336,7 @@ def _score_heuristic(self, heuristic, front_layer, extended_set, layout, swap_qu
 
 def _transform_gate_for_layout(op_node, layout):
     """Return node implementing a virtual op on given layout."""
-    mapped_op_node = deepcopy(op_node)
+    mapped_op_node = copy(op_node)
 
     device_qreg = op_node.qargs[0].register
     premap_qargs = op_node.qargs