[llvm] [mlir] [mlir][emitc] Arith to EmitC conversion: constants (PR #83798)

[Simon Camphausen via llvm-commits]

https://github.com/simon-camp requested changes to this pull request.

Can you please readd the dropped tests in `const.mlir` file. Besides that this looks good.

https://github.com/llvm/llvm-project/pull/83798