Improve LIM fill_always_executed_in computation

Improve LIM fill_always_executed_in computation

Currently the DOM walk over a loop body does not walk into not
always executed subloops to avoid scalability issues since doing
so makes the walk quadratic in the loop depth.  It turns out this
is not an issue in practice and even with a loop depth of 1800
this function is way off the radar.

So the following patch removes the limitation, replacing it with
a comment.

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

	* tree-ssa-loop-im.c (fill_always_executed_in_1): Walk
	into all subloops.

	* gcc.dg/tree-ssa/ssa-lim-17.c: New testcase.