Assertion hoisting for normal if expressions

* src/pf/optimize.lua (simplify): Enable assertion hoisting for
  assertions phrased as "if A B fail" and similar.
Replace "assert" with "if" and "fail"

* src/pf/codegen.lua:
* src/pf/expand.lua:
* src/pf/optimize.lua: Remove "assert", as it is entirely redundant to
  "if" + "fail".