[Lightning] Retrieve the register corresponding to a register argument

[Marc Nieper-Wißkirchen]

Hi Paulo,

let's assume that `jit_arg_register_p` returns true for a specific
argument IN. How can I retrieve the corresponding register (of type
`jit_gpr_t`)?

For an example, consider the following code:

in1 = arg
in2 = arg
movr R0, in1
addr R0, in2
retr R0

Something like the following should be possible (assuming that
`arg_register_p` returns true for `in1` and `in2`:

in1 = arg
in2 = arg
addr REGISTER_OF(in1), REGISTER_OF(in2)
retr REGISTER_OF(in1)

Thanks,

Marc