* [MLIR] Add support for parsing nGraph tensor type

Initial commit that enables nGraph parsing. It's needed for testing.

* [MLIR] Enable nGraph dialect in ngraph-opt

This PR registers nGraph dialect in ngraph-opt and prepares
nGraph lowering pass for LIT testing, fixing all the related issues.
Among other things, lowering pass has to be turned into a function pass,
dead argument in constructor was removed and `convert-ngraph-to-affine`
flag was added.

* Fix issue with function name and multiple functions

* Extend module_function.mlir lit test

* [MLIR] Add support for parsing nGraph element types

It introduces initial support for parsing nGraph signed/unsigned
integer and floating point data types.

* [MLIR] Improve gather op lowering

This PR interchanges indices and param loops in gather lowering so
that a better memory access patter is generated. Fusion of gather
with other ops is also observed with this change.