[GitHub] Add main LLVM Dialect path to MLIR LLVM PR subscribers

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 llvm#65508 add the team as codeowner.

Author: zero9178

.github/CODEOWNERS

@@ -120,6 +120,7 @@
 
 # mlir-llvm
 /mlir/**/LLVM/ @llvm/pr-subscribers-mlir-llvm
+/mlir/**/LLVMIR/ @llvm/pr-subscribers-mlir-llvm
 
 # mlir-linalg
 /mlir/**/*Linalg @llvm/pr-subscribers-mlir-linalg