[llvm] [LoongArch] Optimize extractelement containing variable index for lsx (PR #156792)

[via llvm-commits]

heiher wrote:

> Very little regression. [tangaac/loong-opt-cov-ts at 9805f6b](https://github.com/tangaac/loong-opt-cov-ts/commit/9805f6b186621f58d05a1f699ffe62503c343abb)

So do we need to handle the result differently depending on whether it goes into a GPR or a VR?

https://github.com/llvm/llvm-project/pull/156792