[llvm] [RISCV][TTI] Cost a subvector extract at a register boundary with exact vlen (PR #82405)

[Philip Reames via llvm-commits]

preames wrote:

I forgot a case here, and need to rework this patch.  Specifically, extracting an m2 subvector which is only m1 aligned is not a subregister extract.  It might be two vmv1v, but that's a different case.

https://github.com/llvm/llvm-project/pull/82405