[llvm] [LoongArch] Lower build_vector to broadcast load if possible (PR #135896)

[via llvm-commits]

tangaac wrote:

Here are the files optimized by this pr on llvm-test-suite:
https://github.com/tangaac/loong-opt-cov-ts/commit/9910a8a2f607b26f2c065892d4fbb7a0be1c994e

https://github.com/llvm/llvm-project/pull/135896