Fix compute-predecessors bug in frame.scm

* module/system/vm/frame.scm (compute-predecessors): Fix bug in
  resolving targets of backwards branches.
Fix off-by-one in dump-dfg

* module/language/cps/dfg.scm (dump-dfg): Fix bug where the last
  continuation wasn't printed.
Fix source-line-for-user for unknown line

* module/system/vm/debug.scm (source-line-for-user): Fix to allow for
  unknown lines.
Add $branch expression type

* module/language/cps.scm ($branch): New expression type; will replace
  $kif.

* module/language/cps/arities.scm:
* module/language/cps/closure-conversion.scm:
* module/language/cps/compile-bytecode.scm:
* module/language/cps/cse.scm:
* module/language/cps/dce.scm:
* module/language/cps/dfg.scm:
* module/language/cps/effects-analysis.scm:
* module/language/cps/primitives.scm:
* module/language/cps/renumber.scm:
* module/language/cps/self-references.scm:
* module/language/cps/simplify.scm:
* module/language/cps/slot-allocation.scm:
* module/language/cps/type-fold.scm:
* module/language/cps/types.scm:
* module/language/cps/verify.scm: Adapt to $branch expression type.
CPS conversion produces $branch nodes, not $kif

* module/language/tree-il/compile-cps.scm (unbound?, convert): Create
  $branch nodes instead of $kif nodes.
Remove $kif

* module/language/cps.scm: Remove $kif.

* module/language/cps/compile-bytecode.scm:
* module/language/cps/cse.scm:
* module/language/cps/dce.scm:
* module/language/cps/dfg.scm:
* module/language/cps/effects-analysis.scm:
* module/language/cps/prune-top-level-scopes.scm:
* module/language/cps/renumber.scm:
* module/language/cps/simplify.scm:
* module/language/cps/slot-allocation.scm:
* module/language/cps/type-fold.scm:
* module/language/cps/types.scm:
* module/language/cps/verify.scm: Adapt.