[SLP] Compute a shuffle mask for getGatherCost (#85330)

This is the second of a series of small patches to compute shuffle masks
for the couple of cases where we call getShuffleCost without one. My
goal is to add an invariant that all calls to getShuffleCost for fixed
length vectors have a mask.

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Co-authored-by: Alexey Bataev <a.bataev@gmx.com>