[mlir][linalg] Verify every LinalgOp has a body.

After removing the last LinalgOps that have no region attached we can verify there is a region. The patch performs the following changes:
- Move the SingleBlockImplicitTerminator trait further up the the structured op base class.
- Adapt the LinalgOp verification since the trait only check if there is 0 or 1 block.
- Introduce a getBlock method on the LinalgOp interface.
- Access the LinalgOp body using either getBlock() or getBody() if the concrete operation type is known.

This patch is a follow up to https://reviews.llvm.org/D111233.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D111393