[llvm] [MIPS] [MSA] Widen v2i8, v216 and v2i32 vectors (PR #123040)

[YunQiang Su via llvm-commits]

wzssyqa wrote:

Can we split 
```
Widen v2i8, v2i16 and v2i32 vectors so they don't cast back and forth, and make sure that instructions with correct data unit is being used.
```
and 
```
Handle undef indices for VSHF when lowering VECTOR_SHUFFLE (it crashes if such index is present).
```

https://github.com/llvm/llvm-project/pull/123040