ast: broken phi in loops

[ZingBallyhoo]

can be seen on the created ast in https://github.com/Washi1337/Echo/blob/master/test/Core/Echo.Ast.Tests/AstParserTest.cs#L293

blue box: variable is assigned twice
red box: use of undefined variable

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I am also seeing a KeyNotFoundException at https://github.com/Washi1337/Echo/blob/master/src/Core/Echo.Ast/Construction/BlockTransformer.cs#L199 when parsing some asts. I've repalced this with _context.IncrementVariableVersion(variable) locally but I don't know if this is the correct fix