Introduce 'unreachable' LIMPLE operator

Introduce 'unreachable' LIMPLE operator

Introduce 'unreachable' as LIMPLE operater so we can handle correctly
in the CFG functions throwing values or signaling errors.

	* src/comp.c (retrive_block): Better error diagnostic.
	(emit_limple_insn): Add `unreachable'.
	(compile_function): Fix block iteration.
	(syms_of_comp): Define 'Qunreachable'.
	* lisp/emacs-lisp/comp.el (comp-block): New variable.
	(comp-block-lap): Add `non-ret-insn' slot.
	(comp-branch-op-p): New predicate.
	(comp-limple-lock-keywords): Color `unreachable' as red.
	(comp-compute-edges): Add `unreachable'.
	(comp-fwprop-call): Store non returning function call.
	(comp-fwprop*): Update.
	(comp-clean-orphan-blocks, comp-rewrite-non-locals): New functions.
	(comp-fwprop): Call `comp-rewrite-non-locals'.
	* test/src/comp-tests.el (comp-tests-type-spec-tests): Add two
	tests.
	* test/src/comp-test-funcs.el (comp-test-non-local-1)
	(comp-test-non-local-2, comp-test-non-local-3)
	(comp-test-non-local-4): New functions.