[llvm] [LoongArch] Custom lower vecreduce. (PR #155196)

[via llvm-commits]

tangaac wrote:

lsx https://github.com/tangaac/loong-opt-cov-ts/commit/6978241b2d6f413567cfb274793a417fc0d05ee2
lasx https://github.com/tangaac/loong-opt-cov-ts/commit/ef33bb3239ee4d505345fe09482a15f38e8b0db1

https://github.com/llvm/llvm-project/pull/155196