tree-optimization/107407 - wrong code with DSE

So what happens is that we elide processing of this check with

          /* In addition to kills we can remove defs whose only use
             is another def in defs.  That can only ever be PHIs of which
             we track two for simplicity reasons, the first and last in
             {first,last}_phi_def (we fail for multiple PHIs anyways).
             We can also ignore defs that feed only into
             already visited PHIs.  */
          else if (single_imm_use (vdef, &use_p, &use_stmt)
                   && (use_stmt == first_phi_def
                       || use_stmt == last_phi_def
                       || (gimple_code (use_stmt) == GIMPLE_PHI
                           && bitmap_bit_p (visited,
                                            SSA_NAME_VERSION
                                              (PHI_RESULT (use_stmt))))))

where we have the last PHI being the outer loop virtual PHI and the first
PHI being the loop exit PHI of the outer loop and we've already processed
the single immediate use of the outer loop PHI, the inner loop PHI.  But
we still have to perform the above check!

It's easiest to perform the check when we visit the PHI node instead of
delaying it to the actual processing loop.

	PR tree-optimization/107407
	* tree-ssa-dse.cc (dse_classify_store): Perform backedge
	varying index check when collecting PHI uses rather than
	after optimizing processing of the candidate defs.

	* gcc.dg/torture/pr107407.c: New testcase.