2. Instruction 1 must come first, or it will read the new value instead of the old.
# Write after Write (WAW or "Output"): Two instructions both write the same location. They must occur in their original order.

Technically, there is a fourth type, Read after Read (RAR or "Input"): Both instructions read the same location. Input dependence does not constrain the execution order of two statements, but it is useful in scalar replacement of array elements.

To make sure we respect the three types of dependencies, we construct

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