How do I use the IDIV instruction? (also: Reserving the AX register)

The [IDIV](http://www.felixcloutier.com/x86/IDIV.html) instruction puts its result in the AX register. The problem I have is that I can't figure out how to prevent `GeneralPurposeRegister64` from allocating AX, so my pseudoregisters end up overwriting the input and result.

How do I prevent pseudoregisters from allocating AX?