Wrong single step for IX/IY instructions

[giomba]

Single-stepping an IX or IY related instruction, takes multiple steps, which is somewhat not desired.
Personally I don't know the Z80 enough, maybe this is just the "normal" way it executes code, and thus the way it has been emulated by the underlying library.
But if this is the case, I expect the underlying Z80 emulation library to provide a way to step atomically an IX or IY-related instruction.

Overall, this is just uncomfortable, but does not affect the functionality of the emulator.

Expected behaviour:

• reach an IX or IY related instruction
• step
• address of next instruction is loaded into PC

Actual behavior:

• reach an IX or IY related instruction
• step
• PC still points to the same instruction
• step
• address of next instruction is loaded into PC