[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.

This reverts commit 3f22547f and reland 973e133b with a workaround for
a gcc bug that does not accept lambda default parameters:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59949

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