[mlir][Linalg] Improve region support in Linalg ops.

This revision takes advantage of the newly extended `ref` directive in assembly format
to allow better region handling for LinalgOps. Specifically, FillOp and CopyOp now build their regions explicitly which allows retiring older behavior that relied on specific op knowledge in both lowering to loops and vectorization.

Differential Revision: https://reviews.llvm.org/D96598