Instead of taking an operand and a register, mov just uses the stack
for the operand. Therefore, there's no need for a register member in
inst_t.
As a result, PUSH_*_REGISTER now uses the operand for the register.
Introduced an enum (opcode_type_t) for the masks and values of each
opcode, which allows defining a single enum which checks an opcode by
a opcode_type_t.