[llvm] [AArch64] - Improve costing for Identity shuffles for SVE targets. (PR #165375)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Hi. Can you either move the existing check for identity masks up or move the segmented-sve shuffles down below it?

Also adding a direct test for the cost in llvm/test/Analysis/CostModel/AArch64/ would be useful, and the test looks like it could be cleaned up a bit.

https://github.com/llvm/llvm-project/pull/165375