[llvm] [AArch64][CostModel] Add NFC tests for extractelement cost (PR #108941)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

Some of the half tests look like they should already be higher for fp16. It looks like you are hoping to adjust the cost of fmul lane operations?

These look OK to me, Hopefully the full patch won't require any adjustments. LGTM.

https://github.com/llvm/llvm-project/pull/108941