yosys_optimizer: Directly convert MLIR to Yosys' AST instead of using Verilog

[j2kun]

The YosysOptimizer uses the Verilog Emitter to emit code that can be passed to Yosys, then runs a yosys script. It would be a better practice to directly convert to the RTLIL IR and skip that intermediate step.

[github-actions[bot]]

This issue has 1 outstanding TODOs:

• lib/Transforms/YosysOptimizer/YosysOptimizer.cpp:414: Directly convert MLIR to Yosys' AST instead of using Verilog.

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