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

[Philip Reames via llvm-commits]

preames 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

You did nothing wrong here.  This was entirely my error.

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