tree-optimization/33315 - common stores during sinking

tree-optimization/33315 - common stores during sinking

This implements commoning of stores to a common successor in
a simple ad-hoc way.  I've decided to put it into the code sinking
pass since, well, it sinks stores.  It's still separate since
it does not really sink code into less executed places.

It's ad-hoc since it does not perform any dataflow or alias analysis
but simply only considers trailing stores in a block, iteratively
though.  If the stores are from different values a PHI node is
inserted to merge them.  gcc.dg/tree-ssa/split-path-7.c shows
that path splitting will eventually undo this very transform,
I've decided to not bother with it and simply disable sinking for
the particular testcase.

Doing this transform is good for code size when the stores are
from constants, once we have to insert PHIs the situation becomes
less clear but it's a transform we do elsewhere as well
(cselim for one), and reversing the transform should be easy.

2020-05-15  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/33315
	* tree-ssa-sink.c: Include tree-eh.h.
	(sink_stats): Add commoned member.
	(sink_common_stores_to_bb): New function implementing store
	commoning by sinking to the successor.
	(sink_code_in_bb): Call it, pass down TODO_cleanup_cfg returned.
	(pass_sink_code::execute): Likewise.  Record commoned stores
	in statistics.

	* gcc.dg/tree-ssa/ssa-sink-13.c: New testcase.
	* gcc.dg/tree-ssa/ssa-sink-14.c: Likewise.
	* gcc.dg/tree-ssa/split-path-7.c: Disable sinking.