[SCFToCalyx] Evaluate usefulness of `scf.execute_region` lowering

https://github.com/llvm/circt/pull/1846 introduced lowering of `scf.execute_region` ops to Calyx. However, doubts remain as to whether this is an integral part of the lowering pass, or if it will end up as dead code. As such, we need to evaluate whether there are potential users for the lowering.

Polygeist might be a potential user for `scf.execute_region` lowering. An example is when building affine loops: https://github.com/wsmoses/Polygeist/blob/main/mlir-clang/Lib/clang-mlir.cc#L794.

If we find that `scf.execute_region` are common in lowered C code, then support for lowering it should be kept. If not, it should be removed.

Start by reviewing https://github.com/llvm/circt/pull/1846 and the Polygeist example in mlir-clang/Lib/clang-mlir.cc around line 794. Check whether lowered C code commonly produces scf.execute_region operations and whether Polygeist needs this path. Done means documenting the finding and deciding whether to retain or remove the lowering.

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