[HandshakeOptimizeBitwidths] Fix subi bitwidth calculation#930
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The previous logic was incorrect in the case of mixed `extui` and `extsi` operands, using one bit too little to perfrom the calculation. This PR tries to fix these holes in the `subi` logic while also keeping it as optimized as possible by reusing as much of the logic of `addi` when it applies and special casing the more optimized case when both extension types match. Quite a lot of time was spent writing down somewhat of a proof of correctness in addition to testing in alive2. Fixes #927
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Maybe the proof can be shortened a bit? Here is my attempt for one of the subcases 
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The previous logic was incorrect in the case of mixed
extuiandextsioperands, using one bit too little to perfrom the calculation. This PR tries to fix these holes in thesubilogic while also keeping it as optimized as possible by reusing as much of the logic ofaddiwhen it applies and special casing the more optimized case when both extension types match. Quite a lot of time was spent writing down somewhat of a proof of correctness in addition to testing in alive2.Fixes #927