reshard: fix reverse-iterator direction in getGatheringAxes

[EylonKrause]

Problem

CollectiveInserter::getGatheringAxes (shardy/dialect/sdy/transforms/export/reshard_to_collectives.cc) over-gathers a dimension during reshard lowering: it can return the entire axis list (including axes still present in the output sharding) instead of only the trailing axes absent from the output.

Root cause

The suffix scan starts at inAxes.rbegin() and does --axisRevIt, which advances the std::list reverse_iterator toward the back (off the end) rather than toward the front:

auto axisRevIt = inAxes.rbegin();
while (axisRevIt != inAxes.rend() &&
       !outAxisToDimAndIndex.contains(*axisRevIt)) {
  inAxisSet.erase(*axisRevIt);
  --axisRevIt;   // wrong direction
}

For the function's own documented example (inAxesPerDim = [[], ["x","y","z","w"]], outAxesPerDim = [["y"], []], expected ["z","w"]), -- stops after one step and axisRevIt.base() lands at the front, so the tail loop gathers ["x","y","z","w"] — wrongly gathering "y", which the output still needs — and leaves currentAxes / inAxisSet inconsistent.

The sibling reverse scan in getAllToAllInfo (line ~1134) and popBackFromCurrentAxes (line 118) both use ++.

Fix

Advance with ++axisRevIt.

Verification

Static reasoning + consistency with the sibling scans; reproduces the documented example. No bazel build available on my machine — happy to add a lit test for a reshard that lowers to an all-gather.

[EylonKrause]

Disclosure: this contribution was authored with an AI coding assistant (Claude) and reviewed before submission.