Fix PRE NEW_SETS guarding

Accesses to NEW_SETS should be properly guarded.  Committed
as obvious.

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

	PR tree-optimization/97623
	* tree-ssa-pre.c (create_expression_by_pieces): Guard
	NEW_SETS access.
	(insert_into_preds_of_block): Likewise.