[SVE][NFC] Add SVE cost model tests for gathers/scatters

We previously didn't have any tests to defend the cost model
for gathers and scatters using SVE without a vscale_range
attribute. I've added tests to existing files:

  Analysis/CostModel/AArch64/sve-gather.ll
  Analysis/CostModel/AArch64/sve-scatter.ll

Differential Revision: https://reviews.llvm.org/D109055