[llvm] [mlir] Fix nested codeowners for vector dialect (PR #115473)

[Diego Caballero via llvm-commits]

dcaballe wrote:

Yeah, I also went through this issue a few times. My opinion is that we should limit the rules to dialects or well-defined coarser-grained components. Having rules per .h/.cpp file or small subfolders is too fine grained and introduces too much noise in the CODEOWNERS file.

https://github.com/llvm/llvm-project/pull/115473