v1.15.2

• The placeholder pruning added in 1.15.1 no longer runs on every encoded
  condition.

  Rewriting field == ? bound to null into field IS NULL leaves that
  parameter unreferenced, and 1.15.1 found it by materializing the encoded
  output and scanning it for every placeholder. That cost was paid by every
  query, including the overwhelmingly common one that compares nothing against
  null: ~0.30us per encoded condition (measured on a 3-placeholder
  condition), against the ~0.90us of a whole logged route call after the 1.15.0
  dispatch work.

  Only a placeholder actually rewritten to IS NULL/IS NOT NULL can become
  unreferenced, so those keys are now recorded as they are written, and a
  condition that compares nothing against null returns immediately — without
  materializing the output or scanning it. The output is materialized lazily
  even then, since a recorded key may still be referenced by another operator.

  No behaviour change: same statements, same bound parameters.