Previous loc information on assembly listing was being printed for
every line, while the intent was to print it only when it changes.

Also update the tests to expose and test that case, and remaster
other tests to match.

Disassembly reports generated by pprof -disasm will now include line number
information as generated by objdump. This will make the generated assembly more
readable.

As part of this I've introduced a new assemblyInstruction struct. Previously
the code was reusing the graph.Node to represent assembly instructions but it
seems better to have a dedicated type for this.

Also update the build instructions to use go get