[LLVM] Fix constraint for the atomic: Only assignment is supported for CodegenAtomicStatement

[pramodk]

See the https://github.com/BlueBrain/nmodl/pull/645#issuecomment-1095075632:

libc++abi: terminating with uncaught exception of type std::runtime_error:
              Error: only assignment is supported for CodegenAtomicStatement

The reason is that mech->ion_ina[ion_ina_id] += mech->ina[id] uses + with CodegenAtomicStatement.

As mentioned there, a += b can be written as a = a + b, and hence this error is not quite right.

Currently, we have to write verbose code due to this error message. Maybe this could be fixed with minimum effort?

[pramodk]

By the way, this is not a blocker!