[LoopVectorize][AArch64] Add vectoriser cost model tests for gathers/scatters

I've added some tests that were previously missing for the gather-scatter costs
being calculated by the vectorizer for AArch64:

  Transforms/LoopVectorize/AArch64/sve-gather-scatter-cost.ll

The costs are sometimes different to the ones in

  Analysis/CostModel/AArch64/sve-gather.ll

because the vectorizer also adds on the address computation cost.