rewrite undefined overflow to defined in ifcombine

rewrite undefined overflow to defined in ifcombine

When we make stmts to execute unconditionally in ifcombine we have
to make sure to rewrite stmts that can invoke undefined behavior
on overflow into a form with defined overflow.  That's possible
for all but signed division for which we have to avoid the transform.

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

	* tree-ssa-ifcombine.cc (bb_no_side_effects_p): Avoid executing
	divisions with undefined overflow unconditionally.
	(pass_tree_ifcombine::execute): Rewrite stmts with undefined
	overflow to defined.