[VectorCombine] try to create vector loads from scalar loads

This patch was adjusted to match the most basic pattern that starts with an insertelement
(so there's no extract created here). Hopefully, that removes any concern about
interfering with other passes. Ie, the transform should almost always be profitable.

We could make an argument that this could be part of canonicalization, but we
conservatively try not to create vector ops from scalar ops in passes like instcombine.

If the transform is not profitable, the backend should be able to re-scalarize the load.

Differential Revision: https://reviews.llvm.org/D81766