This works for Eor and Sub.

this also required a rewrite of the perform_memory_operand logic which
should be cleaner now.

This also disables the memcombine optimisation if unaligned stores are
not supported by the backend.
We may add some tests in the future to reenable this in cases where we
can prove that the combined store is aligned.

We should continue here as this is probably just an unsigned loop going
backwards (firm normalizes -1 to +0xFFFFFFFF on unsigned modes). How to
fix this properly is left as an exercise for the next developer :)

This fixes opt/fehler240.c.

This fixes opt/fehler241.c.

This is necessary for ia32 calling conventions for complex float numbers.