Add missing unit dependence vector in data dependence analysis

Add missing unit dependence vector in data dependence analysis

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.

2020-05-13  Bin Cheng  <bin.cheng@linux.alibaba.com>
PR tree-optimization/94969

gcc/
    * tree-data-dependence.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/
    * gcc.dg/tree-ssa/pr94969.c: New test.