This PR moves vector.insert canonicalizers for DenseElementsAttr (splat
and non splat case) to folders. Folders are local, and it's always
better to implement a folder than a canonicalizer.
This PR is mostly NFC-ish, because the functionality mostly remains
same, but is now run as part of a folder, which is why some tests are
changed, because GreedyPatternRewriter tries to fold by default.