tree-optimization/99412 - reassoc and reduction chains

tree-optimization/99412 - reassoc and reduction chains

With -ffast-math we end up associating reduction chains and break
them - this is because of old code that tries to rectify reductions
into a shape likened by the vectorizer.  Nowadays the rank compute
produces correct association for reduction chains and the vectorizer
has robust support to fall back to a regular reductions (via
reduction path) when it turns out to be not a proper reduction chain.

So this patch removes the special code in reassoc which makes
the TSVC s352 vectorized with -Ofast (it is already without
-ffast-math).

	PR tree-optimization/99412
	* tree-ssa-reassoc.cc (is_phi_for_stmt): Remove.
	(swap_ops_for_binary_stmt): Remove reduction handling.
	(rewrite_expr_tree_parallel): Adjust.
	(reassociate_bb): Likewise.
	* tree-parloops.cc (build_new_reduction): Handle MINUS_EXPR.

	* gcc.dg/vect/pr99412.c: New testcase.
	* gcc.dg/tree-ssa/reassoc-47.c: Adjust comment.
	* gcc.dg/tree-ssa/reassoc-48.c: Remove.