[SelectionDAG] Use getVectorElementPointer in DAGCombiner::replaceStoreOfInsertLoad. (#74249)

This ensures we clip the index to be in bounds of the vector we are
inserting into. If the index is out of bounds the results of the insert
element is poison. If we don't clip the index we can write memory that
was not part of the original store.

Fixes #74248.