Chris gave a great explanation of how they relate to each other!
Dependency-wise, XLA depends on LLVM for its CPU and GPU backends, and MLIR is a meta-level IR that is capable of representing LLVM, SIL, the TensorFlow graph, and many many other compiler IRs. MLIR is the next-generation compiler technology that provides shared tooling among different compilers that are implemented using it, and makes it significantly easier to convert between different representations.