Eliminating array bounds checks

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The Oberon compiler inserts code into every array access and every pointer dereference to check for runtime errors, like a subscript that is out of bounds or a pointer that is null. In many cases, it is possible to eliminate the checks because it is possible to determine from the program that no error can occur. For example, an array access inside a FOR loop may be safe given the bounds of the loop, and several uses of the same pointer in successive statements may be able to share one check that the pointer is non-null. Modify the Oberon compiler (or a simpler one taken from the Compilers labs) so that it represents the checks explicitly in its IR, and introduce a pass that removes unnecessary checks, so speeding up the code without compromising safety.

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