* [MLIR] Bump MLIR repo to commit c636f12, 08/09/2019

MLIR Commit:
commit c636f127ee412ef7279ec0d550f42740824cd9ea
Author: Alex Zinenko <zinenko@google.com>
Date:   Fri Aug 9 08:59:45 2019 -0700

    LLVM dialect and translation: support global strings

LLVM Commit:
commit c636f127ee412ef7279ec0d550f42740824cd9ea
Author: Alex Zinenko <zinenko@google.com>
Date:   Fri Aug 9 08:59:45 2019 -0700

    LLVM dialect and translation: support global strings

* [MLIR] Set optimization level for LLVM optimizer and codegen

Now both LLVM optimizer and codegen are aligned with
"NGRAPH_MLIR_OPT_LEVEL" macro.

* [MLIR] Enable LLVM vectorization by initializing TTI

This is the final piece to enable LLVM vectorization for MLIR compiler.
The PR refactors the creation of a target machine in MLIRCompiler so that
we can use it to initialize TargetTransformInfo with the proper host
features and LLVM Loop Vectorizer can get the right vector register
information of the target CPU.