[mlir][spirv][NFC] Clean up scf-to-spirv pass

This is a clean up before fixing issues identified in this pass by
https://github.com/llvm/llvm-project/issues/61380 and similar issues.

- Move patterns definitions closer to declarations.
- Simplify pattern definitions.
- Drop hand-written pass constructor in favor of an auto-generated on.
- Fix typos in pass description.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D146077