OP_ARGCHECK: use custom aux struct

OP_ARGCHECK: use custom aux struct

This op is of class OP_UNOP_AUX, Ops of this class have an op_aux pointer
which typically points to a variable-length malloced array of IVs,
UVs, etc. However in the specific case of OP_ARGCHECK the data stored
in the aux struct is fixed. So this commit casts the aux pointer to a
struct containing the relevant fields (number of parameters etc), rather
than referring to them as aux[0], aux[1] etc. This makes the code more
readable.

Should be no functional changes.