MLIRX is MLIR with extensions. This repository is always meant to be in sync with upstream LLVM, and includes non-trivial extensions to its sub-project MLIR that are meant for advanced research and experimentation. LLVM project components in this repository other than MLIR almost never deviate from upstream. They currently do not.
We believe that MLIR is still in really early stages of its evolution as far as its code generation and optimization infrastructure goes, and there isn't yet a community large enough to provide timely and meaningful reviews upstream to allow rapid progress and experimentation. Most of the extensions in MLIRX are thus planned for future contribution upstream when the time is right. MLIRX does not deviate from MLIR on the core IR support infrastructure part, but primarily provides additional ops and 'mid-level' optimization abstractions, utilities, and passes supporting its polyhedral form.