Lower "not" to "if"

* src/pf/expand.lua (expand_bool): Expand "not" into "if".
* src/pf/codegen.lua (compile_bool): Remove "not" codegen case.
* src/pf/optimize.lua (simplify, simplify_if, cfold, infer_ranges):
  Remove "not" cases.  Add a case for a "not"-like "if" in test
  position.