[wp] factorize & tune union warning

src/plugins/wp/Changelog

src/plugins/wp/MemMemory.ml

src/plugins/wp/MemMemory.mli

src/plugins/wp/MemTyped.ml

src/plugins/wp/MemVar.ml

src/plugins/wp/tests/wp_acsl/oracle/init_value.0.res.oracle

src/plugins/wp/tests/wp_acsl/oracle/init_value.1.res.oracle

...ns/wp/tests/wp_acsl/oracle_qualif/init_value.0.res.oracle

...ns/wp/tests/wp_acsl/oracle_qualif/init_value.1.res.oracle

src/plugins/wp/tests/wp_typed/oracle/cast_fits.0.res.oracle

src/plugins/wp/tests/wp_typed/oracle/cast_fits.1.res.oracle

...gins/wp/tests/wp_typed/oracle_qualif/cast_fits.res.oracle