[SelectionDAG] Extend WidenVecOp_INSERT_SUBVECTOR to cover more cases.

WidenVecOp_INSERT_SUBVECTOR only supported cases where widening
effectively converts the insert into a copy.  However, when the
widened subvector is no bigger than the vector being inserted into
and we can be sure there's no loss of data, we can simply emit
another INSERT_SUBVECTOR.

Fixes: #54982

Differential Revision: https://reviews.llvm.org/D127508