In the reordered RHS path of matchesShlZExt, the code never checked that
each shift amount (0, Stride, 2×Stride, …) appears at most once. When
the same shift appeared in multiple lanes, it still filled Order,
producing a non-permutation (e.g. Order = [0,0,0,1]). That led to bad
shuffle masks and miscompilation (e.g. shuffles with poison).
The patch adds an explicit duplicate check: before setting Order[Idx] =
Pos, it ensures Pos has not been seen before, using a SmallBitVector
SeenPositions(VF). If a position is seen twice, the function returns
false and the optimization is not applied.
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