[llvm] [LegalizeTypes] Handle non byte-sized elt types when splitting INSERT/EXTRACT_VECTOR_ELT (PR #93357)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> A target could just promote the input vector elements itself if that is feasible when doing CustomLowerNode the first time. 

For non-MVT types, I don't think there's any way for a target to request custom lowering.  At least, not using setOperationAction.

https://github.com/llvm/llvm-project/pull/93357