Fix conv skip in gen_lowered_64bit_shifts() (still necessary at all?).
Fix conv skip in gen_lowered_64bit_shifts() (still necessary at all?).

[r21669]

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