[GitHub] Add main LLVM Dialect path to MLIR LLVM PR subscribers (#65513)

The LLVM Dialect in MLIR, which the `mlir-llvm` team is supposed to
provide notifications for, is 98% not nested in a directory called LLVM
but rather LLVMIR. The former only contains some tests.

This should make PRs such as
https://github.com/llvm/llvm-project/pull/65508 add the team as
codeowner.