Make the PR99220 fix more robust

Make the PR99220 fix more robust

This avoids interleaving pattern recognition and load redistribution
optimization since the load_map used in the latter is fragile with
respect to release and reuse of SLP nodes, something which can also
occur within the pattern recognition machinery.

2021-02-25  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (optimize_load_redistribution_1): Delay
	load_map population.
	(vect_match_slp_patterns_2): Revert part of last change.
	(vect_analyze_slp): Do not interleave optimize_load_redistribution
	with pattern detection but do it afterwards.  Dump the
	whole SLP graph after pattern recognition and load
	redistribution optimization finished.