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

[Craig Topper via llvm-commits]

topperc wrote:

@reames I think this commit is causing a failure on test/Analysis/CostModel/RISCV/rvv-vectorextract.ll ? I guess @lukel97  added that test after you made this PR?

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