[llvm] [AArch64] Change cost of (s|z)ext <4 x i8> -> <4 x i32> to 2. (PR #141029)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

One thing to note separately is that the costs from the custom tables are only used for RThru costs, the others default to 1, which is not really accurate either, but that's the same for all entries in the lookup tables

https://github.com/llvm/llvm-project/pull/141029