tree-optimization/95679 - properly signal changes from propagate_into_phi_args

This restores a lost setting of something_changed with the
recent refactoring of the substitute and fold engine.  The
reported ICE in the PR was meanwhile mitigated in other ways
but the issue can still result in missed optimizations via
failed runs of CFG cleanup.

2020-07-29  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95679
	* tree-ssa-propagate.h
	(substitute_and_fold_engine::propagate_into_phi_args): Return
	whether anything changed.
	* tree-ssa-propagate.c
	(substitute_and_fold_engine::propagate_into_phi_args): Likewise.
	(substitute_and_fold_dom_walker::before_dom_children): Update
	something_changed.