ldstopt: fix corner case in phi(store, store) opt
ldstopt: fix corner case in phi(store, store) opt

We first have to create the new replacement store after the phi, so the
following removal of the old nodes does not assume the ptr input value is
unused and kills it.

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