[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.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/analysis/lattices/inverted.h b/src/analysis/lattices/inverted.h
index b232d89aa..22e326742 100644
--- a/src/analysis/lattices/inverted.h
+++ b/src/analysis/lattices/inverted.h
@@ -37,10 +37,14 @@ template<FullLattice L> struct Inverted {
   LatticeComparison compare(const Element& a, const Element& b) const noexcept {
     return lattice.compare(b, a);
   }
-  bool join(Element& joinee, Element joiner) const noexcept {
+
+  template<typename Elem>
+  bool join(Element& joinee, const Elem& joiner) const noexcept {
     return lattice.meet(joinee, joiner);
   }
-  bool meet(Element& meetee, Element meeter) const noexcept {
+
+  template<typename Elem>
+  bool meet(Element& meetee, const Elem& meeter) const noexcept {
     return lattice.join(meetee, meeter);
   }
 };