Remove get_or_alloc_expression_id

This function is no longer needed.

2022-05-19  Richard Biener  <rguenther@suse.de>

	* tree-ssa-pre.cc (get_or_alloc_expression_id): Remove.
	(add_to_value): Use get_expression_id.
	(bitmap_insert_into_set): Likewise.
	(bitmap_value_insert_into_set): Likewise.