Current data dependence analysis misses unit distant vector if DRs in
DDR have the same invariant access functions.  This adds the vector as
the constant access function case.

Also fix typo in testcase.

Backport from master.

2020-06-20  Bin Cheng  <bin.cheng@linux.alibaba.com>

gcc/
	PR tree-optimization/94969
	* tree-data-ref.c (constant_access_functions): Rename to...
	(invariant_access_functions): ...this.  Add parameter.  Check for
	invariant access function, rather than constant.
	(build_classic_dist_vector): Call above function.
	* tree-loop-distribution.c (pg_add_dependence_edges): Add comment.

gcc/testsuite/
	PR tree-optimization/94969
	* gcc.dg/tree-ssa/pr94969.c: New test.

2020-06-20  Jakub Jelinek  <jakub@redhat.com>

gcc/testsuite/
	PR tree-optimization/95110
	* gcc.dg/tree-ssa/pr94969.c: Swap scan-tree-dump-not arguments.