introduce factories to create SSAInstruction objects git-svn-id: https://wala.svn.sourceforge.net/svnroot/wala/trunk@3433 f5eafffb-2e1d-0410-98e4-8ec43c5233c4
git-svn-id: https://wala.svn.sourceforge.net/svnroot/wala/trunk@2593 f5eafffb-2e1d-0410-98e4-8ec43c5233c4