Refine fix for PR78185, improve LIM for code after inner loops

Refine fix for PR78185, improve LIM for code after inner loops

This refines the fix for PR78185 after understanding that the code
regarding to the comment 'In a loop that is always entered we may
proceed anyway.  But record that we entered it and stop once we leave
it.' was supposed to protect us from leaving possibly infinite inner
loops.  The simpler fix of moving the misplaced stopping code
can then be refined to continue processing when the exited inner
loop is finite, improving invariant motion for cases like in the
added testcase.

2021-09-02  Richard Biener  <rguenther@suse.de>

	* tree-ssa-loop-im.c (fill_always_executed_in_1): Refine
	fix for PR78185 and continue processing when leaving
	finite inner loops.

	* gcc.dg/tree-ssa/ssa-lim-16.c: New testcase.