+/// Iteratively prune unit sets. Prune subsets that are close to the superset,
+/// but with one or two registers removed. We occasionally have registers like
+/// APSR and PC thrown in with the general registers. We also see many
+/// special-purpose register subsets, such as tail-call and Thumb
+/// encodings. Generating all possible overlapping sets is combinatorial and
+/// overkill for modeling pressure. Ideally we could fix this statically in
+/// tablegen by (1) having the target define register classes that only include
+/// the allocatable registers and marking other classes as non-allocatable and
+/// (2) having a way to mark special purpose classes as "don't-care" classes for
+/// the purpose of pressure. However, we make an attempt to handle targets that
+/// are not nicely defined by merging nearly identical register unit sets
+/// statically. This generates smaller tables. Then, dynamically, we adjust the
+/// set limit by filtering the reserved registers.
+///
+/// Merge sets only if the units have the same weight. For example, on ARM,
+/// Q-tuples with ssub index 0 include all S regs but also include D16+. We
+/// should not expand the S set to include D regs.