[MLIR][SPIRV] Control attributes support for loop and selection

This patch handles loopControl and selectionControl in parsing and
printing. In order to reuse the functionality, and avoid handling cases when
`{` of the region is parsed as a dictionary attribute, `control` keyword was
introduced.`None` is a default control attribute. This functionality can be
later extended to `spv.func`.
Also, loopControl and selectionControl can now be (de)serialized.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D84175