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

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heiher wrote:

> I think these cases can reproduce the original issue. Of course, these examples are ultimately converted to i32, so I'm not sure if it's necessary.

Thanks for providing the tests. Since the original LLVM IR inputs differ, I intend to include these test cases as well.

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