Lowering of build using edsc features of MLIR (v2)

[dcrc2]

Updated version of #692, rebasing onto master.

Original description from #692:

This PR showcases the usage of MLIR edsc features in the lowering of a build node.

• The C++ code to emit the IR looks much closer to the IR itself using this approach.
• I choose a lowering target with a higher abstraction level: Lowering to a loop (nest) instead of a chain of basic blocks, i.e scf dialect instead of std dialect.

edsc - Embedded Domain Specific Concepts

We move away from using a series of create calls on the builder itself, i.e. old build lowering Instead, we set up a ScopedContext once with the correct insertion point. Then we can use a much simpler declarative interface to emit new operations. new build lowering

This PR is tested to work partially with LLVM commit f4013359b3da2c78e94a64245de8638460f96c1a. e.g. lowering of maths.ks works as expected ksc-mlir MLIR mlir/test/Programs/maths.ks. However, due to slight changes in the MLIR format all FileCheck tests are expected to fail. Creating new FileCheck tests is eased dramatically by using generate-test-checks.py

[dcrc2]

Several tests were failing in #692 when trying to lower code using scf.for to LLVM. This requires a preparatory pass to convert SCF to std (eqivalent to mlir-opt --convert-scf-to-std --convert-std-to-llvm as described in users/ml/relu3.md). Having fixed this problem, the only failing test is now build.ks for which the expected output has changed.

[dcrc2]

Test expectations are now updated and this PR is ready.