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

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

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

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