ia32: Remove strange case, which allowed the whole register class, when actually no register is allowed.
ia32: Remove strange case, which allowed the whole register class, when actually no register is allowed.

This lets the register allocator fail instead of silently generating invalid code.

jonashaag/libfirm,jonashaag/libfirm,8l/libfirm,davidgiven/libfirm,libfirm/libfirm,davidgiven/libfirm,MatzeB/libfirm,MatzeB/libfirm,davidgiven/libfirm,killbug2004/libfirm,davidgiven/libfirm,8l/libfirm,libfirm/libfirm,killbug2004/libfirm,davidgiven/libfirm,MatzeB/libfirm,8l/libfirm,jonashaag/libfirm,jonashaag/libfirm,libfirm/libfirm,killbug2004/libfirm,8l/libfirm,jonashaag/libfirm,jonashaag/libfirm,libfirm/libfirm,8l/libfirm,MatzeB/libfirm,8l/libfirm,MatzeB/libfirm,MatzeB/libfirm,jonashaag/libfirm,libfirm/libfirm,8l/libfirm,davidgiven/libfirm,killbug2004/libfirm,killbug2004/libfirm,killbug2004/libfirm,killbug2004/libfirm,davidgiven/libfirm,MatzeB/libfirm