[llvm] [LoongArch] Fix instruction selection failure for BUILD_VECTOR on LA32 (PR #158788)

[via llvm-commits]

heiher wrote:

> Is it necessary to add tests for i8/i16?

Happy to add more tests for coverage. Could you show the i8/i16 tests for this case?

https://github.com/llvm/llvm-project/pull/158788