tree-optimization/97901 - ICE propagating out LC PHIs

We need to fold the stmt to canonicalize MEM_REFs which means
we're back to using replace_uses_by.  Which means we need dominators
to not require a CFG cleanup upthread.

2020-11-19  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97901
	* tree-ssa-propagate.c (clean_up_loop_closed_phi): Compute
	dominators and use replace_uses_by.

	* gcc.dg/torture/pr97901.c: New testcase.