Support backends using OpenQasm2

Required prerequisites

• Search the issue tracker to check if your feature has already been mentioned or rejected in other issues.

Describe the feature

We have a pipeline support for backends using OpenQasm2 but bunch of tests fail due to missing optimizations.

• Some unused globals are not removed:

  • custom_operation_basic.cpp
  • custom_operation_adj.cpp
  • int8_t.cpp
  • int8_t_free_func.cpp
  • load_value.cpp
  • swap_gate.cpp
  • variable_size_qreg.cpp

• https://github.com/NVIDIA/cuda-quantum/pull/2379/

• CCZ and r?<adj> operations need to be decomposed

  • graph_coloring-1.cpp
  • graph-coloring.cpp
  • sudoku_2x2.cpp
  • sudoku_2x2-1.cpp
  • sudoku_2x2-bit_names.cpp
  • sudoku_2x2-reg_name.cpp

• https://github.com/NVIDIA/cuda-quantum/pull/2379

• quake.concat and quake.subveq need expansion

  • custom_operation_ctrl.cpp
  • qspan_slices.cpp

• quake.subveq used in mz(veq)

  • phase-estimation.cpp

• in Python, mz(veq) generates allocations and stores are not optimized away

  • Note that using cudaq.translate API instead of cudaq.sample works.
    Looks like the loop might be created by expand-measurements pass in one case but not in the other.
    Consider optimizing away unused array allocations.

• Add tests for python

• The following generated code is not accepted on some backends, it errors out with
  RuntimeError: [line 10] cannot declare bit register. Only 1 bit register(s) is/are supported:

OPENQASM 2.0;

qreg var0[2];
h var0[0]; cnot var0[0], var0[1];
creg var3[1];
measure var0[0] -> var3[0];
creg var4[1];
measure var0[1] -> var4[0];

Related: https://github.com/NVIDIA/cuda-quantum/issues/2325

Start with the failing C++ cases listed in the issue and review PR 2379, then compare the cudaq.translate and cudaq.sample paths for Python measurement handling. Inspect the expand-measurements pass and the generated OpenQASM 2.0 example. Done means the listed optimizations and decompositions work, Python tests exist, and affected backends accept the generated code.