gas: Change the Aggregate instrumentation kind to represent cost as `a*x + b`

[nagisa]

Aggregate instrumentation points can represent a fixed cost to start executing the instruction and then an incremental cost to continue execution based on the size or amplitude of the inputs. For that reason the usual single constant cost is not sufficient for this instrumentation kind.

Furthermore, implementing this would potentially allow merging Aggregate with some other instrumentation kinds, such as would be the case in:

loop
    aggregate
end