tree-optimization/101025 - fix store-motion dependence checking

This plugs a hole in store-motion where it fails to perform dependence
checking on conditionally executed but not store-motioned refs.

2021-06-11  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101025
	* tree-ssa-loop-im.c (sm_seq_valid_bb): Make sure to process
	all refs that require dependence checking.

	* gcc.dg/torture/pr101025.c: New testcase.