supporting assignment of measurement value to variable in autoqasm

[rjain37]

Feature Description

These two autoqasm programs operate the same, but have different ir values:

First:

@aq.main
def my_program():
    ins.h(0)
    ins.cnot(0, 1)
    return ins.measure([0, 1])

print(my_program.build().to_ir())

OPENQASM 3.0;
output bit[2] return_value;
qubit[2] __qubits__;
h __qubits__[0];
cnot __qubits__[0], __qubits__[1];
bit[2] __bit_0__ = "00";
__bit_0__[0] = measure __qubits__[0];
__bit_0__[1] = measure __qubits__[1];
return_value = __bit_0__;

Second:

@aq.main
def my_program():
    ins.h(0)
    ins.cnot(0, 1)
    result = ins.measure([0, 1])
    return result

print(my_program.build().to_ir())

OPENQASM 3.0;
output bit[2] result;
qubit[2] __qubits__;
h __qubits__[0];
cnot __qubits__[0], __qubits__[1];
bit[2] __bit_0__ = "00";
__bit_0__[0] = measure __qubits__[0];
__bit_0__[1] = measure __qubits__[1];
result = __bit_0__;

The first one doesn't have an issue when being converted to QIR, but the second one does.