* [MLIR] Move MLIR compiler files to contrib/mlir/compiler

We'll add contrib/mlir/tools directory in subsequent PRs for ngrpah-opt
and other tools.

* [MLIR] Clean up MLIR related CMake files

This PR prepares the introduction of src/contrib/mlir/tools and LIT testing by
refactoring and cleaning up the common part with src/contrib/mlir/compiler:

- Common code for src/contrib/mlir/tools and src/contrib/mlir/compiler is
moved to src/contrib/mlir/CMakeList.txt.
- More generic code, common for src/contrib/mlir/* and LIT configuration
files, is moved to cmake/external_mlir.cmake with pre-existing similar code.
- Redundant environment var LLVM_DIR is removed in favor of LLVM_CMAKE_PATH.
- Redundant cmake messages are removed.

* Replacing LLVM_CMAKE_PATH with LLVM_DIR