[llvm] [TTI][RISCV]Improve costs for fixed vector whole reg extract/insert. (PR #80164)

[Luke Lau via llvm-commits]

lukel97 wrote:

#81751 handles both fixed and scalable vectors from the looks of things. I wonder if it's possible to have this patch handle the whole reg extract/insert case for scalable vectors too? Any scalable vector extract/insert should be free if both the vector and subvector are >= LMUL 1.

https://github.com/llvm/llvm-project/pull/80164