* Clean unreachable block using dominance tree to handle circularities

* Clean unreachable block using dominance tree to handle circularities

With this commit unreachable basic blocks are pruned automatically by
comp-ssa relying on dominance analysis.  This solves the issue of
unreachable cluster of basic blocks referencing each other.

	* lisp/emacs-lisp/comp.el (comp-block-lap): New `no-ret' slot.
	(comp-compute-dominator-tree): Update.
	(comp-remove-unreachable-blocks): New functions.
	(comp-ssa): Update to call `comp-remove-unreachable-blocks'.
	(comp-clean-orphan-blocks): Delete.
	(comp-rewrite-non-locals): Update and simplify.