tree-optimization/94949 - fix load eliding in SM

This fixes the case of not using the multithreaded model when
only conditionally storing to the destination.  We cannot elide
the load in this case.

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

	PR tree-optimization/94949
	* tree-ssa-loop-im.c (execute_sm): Check whether we use
	the multithreaded model or always compute the stored value
	before eliding a load.

	* gcc.dg/torture/pr94949.c: New testcase.