[analysis] Allow joining a single vector element efficiently (#6071)

Previously, modifying a single vector element of a `Shared<Vector>` element
required materializing a full vector to do the join. When there is just a single
element to update, materializing all the other elements with bottom value is
useless work. Add a `Vector<L>::SingletonElement` utility that represents but
does not materialize a vector with a single non-bottom element and allow it to
be passed to `Vector<L>::join`. Also update `Shared` and `Inverted` so that
`SingletonElement` joins still work on vectors wrapped in those other lattices.

0 files changed, 0 insertions, 0 deletions