tree-optimization/105173 - fix insertion logic in reassoc

tree-optimization/105173 - fix insertion logic in reassoc

The find_insert_point logic around deciding whether to insert
before or after the found insertion point does not handle
the case of _12 = ..;, _12, 1.0 well.  The following puts the
logic into find_insert_point itself instead.

2022-04-06  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105173
	* tree-ssa-reassoc.cc (find_insert_point): Get extra
	insert_before output argument and compute it.
	(insert_stmt_before_use): Adjust.
	(rewrite_expr_tree): Likewise.

	* gcc.dg/pr105173.c: New testcase.