Registers are now just words, with pushing from and moving to
registers with specified subtypes just pushing those types into the
word registers. That means there are 8 word registers which can act
as 16 half word registers, which themselves can act as 64 byte
registers.
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.
