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

DAGTypeLegalizer::SplitVecRes_INSERT_VECTOR_ELT and
DAGTypeLegalizer::SplitVecRes_EXTRACT_VECTOR_ELT did not handle
non byte-sized elements properly. In fact, it only dealt with
elements smaller than 8 bits (as well as byte-sized elements).

This patch generalizes the support for non byte-sized element by
always widening the the vector elements to next "round integer type"
(a power of 2 bit size). This should make sure that we can access a
single element via a simple byte-addressed scalar load/store.

Also removing a suspicious CustomLowerNode call from
SplitVecRes_INSERT_VECTOR_ELT. Considering that it did not reset
the Lo/Hi out arguments before the return I think that
DAGTypeLegalizer::SplitVectorResult could be fooled into registering
the input vector as being the result. This should however not have
caused any problems since DAGTypeLegalizer::SplitVectorResult is
doing the same CustomLowerNode call, making the code removed by
this patch redundant.