[mlir][Vector] Move vector.extract canonicalizers for DenseElementsAttr to folders (#127995)
This PR moves vector.extract canonicalizers for DenseElementsAttr (splat
and non splat case) to folders. Folders are local, and it's always
better to implement a folder than a canonicalization pattern.
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.
There is also a test change which makes the indices of a vector.extract
test dynamic. This is so that it doesn't fold away after this pr.