tree-optimization/98024 - fix rnflow regression

tree-optimization/98024 - fix rnflow regression

The change to make PRE insertion iterate less had a typo in checking
successors RPO state.  Fixing this exposes that regular PRE insertion
when facing a value that is the same on all edges inserts an
assignment in place of a PHI node but fails to set up things so that
this insertion is not repeated (it correctly does not return
'new_stuff').  But with the new iteration scheme this causes us
to repeatedly insert such assignment and change AVAIL_OUT over to
the newly inserted expression.  The fix is to treat this as PHI
and insert into PHI_GEN, avoiding repetitive insertion.

2020-11-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98024
	* tree-ssa-pre.c (insert): Fix successor RPO order check.
	(do_pre_regular_insertion): When inserting an assignment
	in place of an all-same-value PHI still record that into
	PHI_GEN.