rtl-optimization/93946 - fix TBAA for redundant store removal in CSE

rtl-optimization/93946 - fix TBAA for redundant store removal in CSE

It turns out RTL CSE tries to remove redundant stores but fails to
do the usual validity check what such a change is TBAA neutral to
later loads.

This now triggers with the PR93946 testcases on nios2.

2020-04-08  Richard Biener  <rguenther@suse.de>

	PR rtl-optimization/93946
	* cse.c (cse_insn): Record the tabled expression in
	src_related.  Verify a redundant store removal is valid.