[llvm] [LoongArch][NFC] Add tests for vector extract comparison combines (PR #214203)

[via llvm-commits]

heiher wrote:

> A couple of nits:
> 
> * This doesn't seem to be `NFC` since the added patterns change instruction selection/code generation. Please remove `[NFC]` from the title.
> * `vec-extract-brcond.ll` seems unrelated to the comparison combines covered by this patch. Could it be moved to a separate patch?

Thanks! It didn't automatically rebase when the base PR was merged. It's rebased now.

https://github.com/llvm/llvm-project/pull/214203