Clarify wording in a comment.

lib/codegen/src/regalloc/liveness.rs

@@ -42,9 +42,9 @@
 //! This algorithm has some disadvantages that makes us look elsewhere:
 //!
 //! - Quadratic memory use. We need a bit per variable per basic block in the function.
-//! - Sparse representation. In practice, the majority of SSA values never leave their basic block,
-//!   and those that do span basic blocks rarely span a large number of basic blocks. This makes
-//!   the bit-vectors quite sparse.
+//! - Dense representation of sparse data. In practice, the majority of SSA values never leave
+//!   their basic block, and those that do span basic blocks rarely span a large number of basic
+//!   blocks. This makes the data stored in the bitvectors quite sparse.
 //! - Traditionally, the data-flow equations were solved for real program *variables* which does
 //!   not include temporaries used in evaluating expressions. We have an SSA form program which
 //!   blurs the distinction between temporaries and variables. This makes the quadratic memory