[alias] simplified assignment functions

src/plugins/alias/abstract_state.ml

src/plugins/alias/abstract_state.mli

src/plugins/alias/analysis.ml

src/plugins/alias/tests/basic/oracle/assignment2.res.oracle

src/plugins/alias/tests/basic/oracle/assignment3.res.oracle

src/plugins/alias/tests/basic/oracle/assignment4.res.oracle

src/plugins/alias/tests/basic/oracle/assignment5.res.oracle

src/plugins/alias/tests/basic/oracle/conditional2.res.oracle

src/plugins/alias/tests/basic/oracle/function4.res.oracle

src/plugins/alias/tests/offsets/oracle/array1.res.oracle

src/plugins/alias/tests/offsets/oracle/array2.res.oracle

src/plugins/alias/tests/offsets/oracle/array3.res.oracle

src/plugins/alias/tests/offsets/oracle/nested1.res.oracle

src/plugins/alias/tests/offsets/oracle/nested2.res.oracle

src/plugins/alias/tests/offsets/oracle/structure1.res.oracle

src/plugins/alias/tests/offsets/oracle/structure2.res.oracle

src/plugins/alias/tests/offsets/oracle/structure3.res.oracle

src/plugins/alias/tests/offsets/oracle/structure4.res.oracle

src/plugins/alias/tests/offsets/oracle/structure5.res.oracle

...plugins/alias/tests/real_world/oracle/example1.res.oracle