[PATCH PR94969]Add unit distant vector to DDR in case of invariant access functions

[bin.cheng via Gcc-patches]

Hi,
As analyzed in PR94969, data dependence analysis now misses dependence vector 
for specific case in which DRs in DDR have the same invariant access functions. 
 This simple patch fixes the issue by also covering invariant cases.  Bootstrap 
and test on x86_64, is it OK?

Thanks,
bin

2020-05-11  Bin Cheng  <bin.ch...@linux.alibaba.com>

        PR tree-optimization/94969
        * 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
2020-05-11  Bin Cheng  <bin.ch...@linux.alibaba.com>

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

pr94969-20200511.txt
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