[MIPS] [MSA] Widen v2i8, v216 and v2i32 vectors (#123040)
- 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.
- Handle undef indices for VSHF when lowering VECTOR_SHUFFLE (it crashes
if such index is present).
