Identify loops

Identify loops

* module/language/cps/dfg.scm (compute-dom-edges)
  (compute-join-edges, compute-reducible-back-edges)
  (compute-irreducible-dom-levels, compute-nodes-by-level)
  (mark-loop-body, mark-irreducible-loops, identify-loops): Identify
  loops.  Irreducible loops are TODO.

* test-suite/tests/rtl-compilation.test ("contification"): Add an
  irreducible loop test.