[X86] X86DAGToDAGISel - attempt to merge XMM/YMM loads with YMM/ZMM loads of the same ptr (#73126)

If we are loading the same ptr at different vector widths, then reuse the largest load and just extract the low subvector.

Unlike the equivalent VBROADCAST_LOAD/SUBV_BROADCAST_LOAD folds which can occur in DAG, we have to wait until DAGISel otherwise we can hit infinite loops if constant folding recreates the original constant value.

This is mainly useful for better constant sharing.