Implement ProgramOrder for Layout.
Implement ProgramOrder for Layout.

Assign sequence numbers to both instructions and EBB headers such that
their relative position can be determined in constant time simply by
comparing sequence numbers.

Implement the sequence numbers with a scheme similar to the line numbers
used in BASIC programs. Start out with 10, 20, 30, ... and pick numbers
in the gaps as long as possible. Renumber locally when needed.

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