There are some fixes for AVX-512 vectorization.

* Bump MLIR version and minor fixes in nGraph

* advance commit ids

* fix commit id

* [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

* [MLIR] Add config files for LIT

LIT is the LLVM testing tool that is used in LLVM and MLIR. It's
necessary to enable all the MLIR testing in nGraph. This will provide
a complementary finer grain level of testing than the one currently done
with gtest. LIT will allow testing each MLIR-based nGraph transformation
without having to run the whole nGraph pipeline. For example, we will be
able to write an nGraph dialect input, invoke an single MLIR transformation
on it and check that the output is the expected.

This PR adds the basic configuration files to enable LIT tool
in nGraph for MLIR Compiler. `affine_lowering/lit_test.mlir` is a
dummy test to exercise the new testing feature. Lit testing will be
enabled in CMAKE in a subsequent PR.

* Address feedback

* [MLIR] Bump MLIR repo 8/20/2019

MLIR_
commit 0cdb20a6add19bc96c20dad28589a1e54e4d8469
Author: Lei Zhang <antiagainst@google.com>
Date:   Tue Aug 20 13:33:41 2019 -0700

    Add spv.specConstant and spv._reference_of

LLVM:
commit 3b9a27b6908040881dad394022f8c472c15c0784
Author: Simon Pilgrim <llvm-dev@redking.me.uk>
Date:   Tue Aug 20 17:54:37 2019 +0000

    Fix typo in comment. NFCI.

* Address Bob's feedback

* [MLIR] Bump MLIR repo to commit 59167c2, 07/25/2019.

MLIR commit:
Author: River Riddle <riverriddle@google.com>
Date:   Wed Jul 24 16:41:11 2019 -0700

    NFC: Use ValueOfRange instead of T in Diagnostic::appendRange.

        For iterator_range, T is often the name of another iterator type
        and not the the value of the range.

LLVM commit:
Author: Marshall Clow <mclow.lists@gmail.com>
Date:   Thu Jul 25 03:26:05 2019 +0000

    Implement change #4 of P1466: Change weekday to accept both 0 and 7
    as Sunday. Add accessors 'c_encoding' and 'iso_encoding' to provide
    different interpretations of the weekday. Remove 'operator unsigned'

* style

* Move MLIR/LLVM repos a bit more forward

Improvements and changes in dialect conversion APIs. Type conversion
and op rewriting are now a bit more independent mechanisms. Dialect
conversion now requires a target object with the legal transformations
to be applied. Legality checks to detect illegal operations after
dialect conversion have been strengthened significantly.

MLIR LLVM commit:
commit 82d5084cf28bca7a0eebec283cdeae118dfa1d11 (HEAD)
Author: Jacques Pienaar <jpienaar@google.com>
Date:   Wed Jun 26 11:12:40 2019 -0700

    Split out TranslateClParser and add new parse method that reuses
    SourceMgr.

MLIR commit:
commit 82d5084cf28bca7a0eebec283cdeae118dfa1d11 (HEAD)
Author: Jacques Pienaar <jpienaar@google.com>
Date:   Wed Jun 26 11:12:40 2019 -0700

    Split out TranslateClParser and add new parse method that reuses
    SourceMgr.

* Upgrade MLIR. Several code fixes based on API changes

* Fixes due to DialectConv API changes

* style-apply

* PR fixes

* Initial td file. Cmake changes

* Move all ops to .td file.

* Added few more opcodes to show-case. Fixed PR feedback

* Remove NG_ prefix of opcode records. Some fixes

* Added some doc

*  Adding back NG prefix

* Bug fix in MLIR gen

* Create MLIR as cmake external project. Clone and build via ngraph cmake

* Moved code and enabled compilation. Need to clone and build MLIR/LLVM during cmake step, so find_package can work

* clone and build llvm/mlir during configuration. Compiles now. Needs more testing

* Force DEX only if MLIR is ON

* Remove extra cmake file. Style-apply

* Remove redundant files in cpu/mlir

* Update CODEOWNERS. Check for ninja and fail if not found

* Fixing post merge issues