Flatten controlled-CZ and controlled-CX more consistently

[daxfohl]

Fixes #7241 by absorbing the control layer of controlled-CZ into the control itself, leaving a controlled-Z, and does the equivalent for CX / X. As documented in the issue, this approach allows for more consistency in how controlled gates are represented.

As stated in the linked issue, almost all existing controlled-CZ cases already work this way; the only outlier is when control_values==[0]. Eliminating that outlier allows most of the special handling to be consolidated in the base gates (Z and X), so it's already possible to see some benefit from the added consistency.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (e9a592d) to head (cb4ca25).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7365   +/-   ##
=======================================
  Coverage   98.68%   98.68%           
=======================================
  Files        1112     1112           
  Lines       97599    97600    +1     
=======================================
+ Hits        96315    96318    +3     
+ Misses       1284     1282    -2     

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[pavoljuhas]

Nice, thank you for fixing this! Before merging, please apply the patch below to address todo from #7269 (cc @codrut3).

diff --git a/cirq-core/cirq/ops/controlled_gate.py b/cirq-core/cirq/ops/controlled_gate.py
index c07b134b9..915c0805c 100644
--- a/cirq-core/cirq/ops/controlled_gate.py
+++ b/cirq-core/cirq/ops/controlled_gate.py
@@ -151,12 +151,7 @@ class ControlledGate(raw_types.Gate):
         )
         # Prefer the subgate controlled version if available
         if self != controlled_sub_gate:
-            # Prevent 2-cycle from appearing in the recursive decomposition
-            # TODO: Remove after #7241 is resolved
-            if not isinstance(controlled_sub_gate, ControlledGate) or not isinstance(
-                controlled_sub_gate.sub_gate, common_gates.CZPowGate
-            ):
-                return controlled_sub_gate.on(*qubits)
+            return controlled_sub_gate.on(*qubits)
         if (
             protocols.has_unitary(self.sub_gate)
             and protocols.num_qubits(self.sub_gate) == 1