Neither one really has a direct "equivalent" in assembly.

Prefix is going to look something like:

  inc eax # increment the value
  mov eax,ecx # copy the return value to its destination

Postfix is going to look something like:

  mov eax,ebx # copy original value
  inc eax # increment value
  mov ebx,ecx # copy original value to final destination

The actual code you'd get would vary a lot between compilers, architectures, and even just different contexts within the same compiler-architecture combination.