[llvm] [RISCV][GlobalISel] Vector Extension vadd Legalizer (PR #71400)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> > Are you able to reorder your commits with `git rebase -i`? The commits that this PR is stacked on should come first, followed by the commits introduced by this PR. It may also be helpful to reword your commits using the `r` option once you are in `git rebase -i` so that it takes the name of the PR.
> 
> Does it look right now?

Yes, thanks!

https://github.com/llvm/llvm-project/pull/71400