[llvm] [RISCV][TTI] Extract subvector at index zero is free (PR #81751)

[Luke Lau via llvm-commits]

lukel97 wrote:

> LGTM.
> 
> Not related to this patch, how come we end up handling scalable vectors in the shuffle cost anyway? The only valid shufflevector for a scalable vector is a zeroinitializer mask IIUC, i.e. SK_Broadcast

Looks like we reuse getShuffleCost for the vector.insert/vector.extract intrinsics:

https://github.com/llvm/llvm-project/blob/8ce144800a7ed7c1e42343b3a9ac5e0ffdbfddbf/llvm/include/llvm/CodeGen/BasicTTIImpl.h#L1636-L1655

https://github.com/llvm/llvm-project/pull/81751