tree-optimization/97623 - Avoid PRE hoist insertion iteration

tree-optimization/97623 - Avoid PRE hoist insertion iteration

We are not really interested in PRE opportunities exposed by
hoisting but only the other way around.  So this moves hoist
insertion after PRE iteration finished and removes hoist
insertion iteration alltogether.

It also guards access to NEW_SETS properly.

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

	PR tree-optimization/97623
	* tree-ssa-pre.c (insert): Move hoist insertion after PRE
	insertion iteration and do not iterate it.
	(create_expression_by_pieces): Guard NEW_SETS access.
	(insert_into_preds_of_block): Likewise.

	* gcc.dg/tree-ssa/ssa-hoist-3.c: Adjust.
	* gcc.dg/tree-ssa/ssa-hoist-7.c: Likewise.
	* gcc.dg/tree-ssa/ssa-pre-30.c: Likewise.