[VectorCombine] widen a load with subvector insert

This adapts/copies code from the existing fold that allows
widening of load scalar+insert. It can help in IR because
it removes a shuffle, and the backend can already narrow
loads if that is profitable in codegen.

We might be able to consolidate more of the logic, but
handling this basic pattern should be enough to make a small
difference on one of the motivating examples from issue #17113.
The final goal of combining loads on those patterns is not
solved though.

Differential Revision: https://reviews.llvm.org/D137341