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

[Philip Reames via llvm-commits]

https://github.com/preames requested changes to this pull request.

I have posted an alternative patch for the remaining insertsubvector case here: https://github.com/llvm/llvm-project/pull/85240

When writing this, I discovered that this patch models a lowering which is not implemented.  There is a patch on review, but it hasn't landed yet.

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