compiler: Don't let continuation-analysis add a cycle...

compiler: Don't let continuation-analysis add a cycle...

...to the block tree, which it would do (causing a "maximum recursion
depth exceeded" abort during block->dbg-block) when compiling crazy
code like:

      (define (fubar param)
	(define (closure) param)
	(define (loop)
	  (closure)
	  (loop)
	  ;; It doesn't break without this
	  (tail-call)))
      (fubar sumpn)

This patch checks that the "Acceptable substitute" for a block's
stack-link is not inside the block.