tree-optimization/104700 - adjust constant handling in PRE

tree-optimization/104700 - adjust constant handling in PRE

The following refactors find_or_generate_expression to more properly
handle constant valued SSA names thereby simplifying the code and
avoiding ICEing after the last change to NARY processing.

2022-02-28  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104700
	* tree-ssa-pre.cc (get_or_alloc_expr_for): Remove and inline
	into ...
	(find_or_generate_expression): ... here, simplifying code.

	* gcc.dg/pr104700-2.c: New testcase.
	* gcc.dg/torture/pr104700-1.c: Likewise.