If a candidate PRE cannot get the prologue because a block BB is
reachable from it, but PRE does not dominate BB, we try again with the
dominators of PRE.  That "try again" needs to again consider BB though,
we aren't done with it.

	PR rtl-optimization/67778
	PR rtl-optimization/68634
	PR rtl-optimization/68909
	* shrink-wrap.c (try_shrink_wrapping): Add comment.  Don't pop
	block from the stack until done with it.  Remove a superfluous
	bitmap set.  Remove a superfluous bitmap test.

From-SVN: r232148