Added assertions to enforce proper use of set_value, set_store
Added assertions to enforce proper use of set_value, set_store

[r528]

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