Note [Compiler phases]

The CompilerPhase says which phase the simplifier is running in:

* InitialPhase: before all user-visible phases

* Phase 2,1,0: user-visible phases; the phase number
  controls rule ordering an inlining.

* FinalPhase: used for all subsequent simplifier
  runs. By delaying inlining of wrappers to FinalPhase we can
  ensure that RULE have a good chance to fire. See
  Note [Wrapper activation] in GHC.Core.Opt.WorkWrap

  NB: FinalPhase is run repeatedly, not just once.

  NB: users don't have access to InitialPhase or FinalPhase.
  They write {-# INLINE[n] f #