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

[Luke Lau via llvm-commits]

lukel97 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?

My bad, I forgot that this PR would need rebased after adding them

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