Refactor DenseElementsAttr to support auto-splatting the dense data on...

Refactor DenseElementsAttr to support auto-splatting the dense data on construction. This essentially means that we always auto-detect splat data and only store the minimum amount of data necessary. Support for parsing dense splats, and removing SplatElementsAttr(now that it is redundant) will come in followup cls

PiperOrigin-RevId: 252720561