flow-control: Store VariableId instead of VarUsage.

[liorgold2]

Stack:

• flow-control: Some cleanup. #8400
• flow-control: Support let-else. #8399
• flow-control: Small refactor. #8397
• flow-control: Store VariableId instead of VarUsage. #8396 ⬅

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

[reviewable-StarkWare]

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[ilyalesokhin-starkware]

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[ilyalesokhin-starkware]

thanks

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[orizi]

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[orizi]

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