Every simplifier phase can have an arbitrary number of tags and multiple
phases can share the same tags. The tags can be used as arguments to
-ddump-simpl-phases to specify which phases are to be dumped.
For instance, -ddump-simpl-phases=main will dump the output of phases 2, 1 and
0 of the initial simplifier run (they all share the "main" tag) while
-ddump-simpl-phases=main:0 will dump only the output of phase 0 of that run.

At the moment, the supported tags are:

  main                 The main, staged simplifier run (before strictness)
  post-worker-wrapper  After the w/w split
  post-liberate-case   After LiberateCase
  final                Final clean-up run

The names are somewhat arbitrary and will change in the future.