[llvm-cov gcov] Replace Donald B. Johnson's cycle enumeration with iterative cycle finding

gcov computes the line execution count as the sum of (a) counts from

predecessors on other lines and (b) the sum of loop execution counts of blocks

on the same line (think of loops on one line).

For (b), we use Donald B. Johnson's cycle enumeration algorithm and perform

cycle cancelling for each cycle. This number of candidate cycles were

exponential and D93036 made it polynomial by skipping zero count cycles. The

time complexity is high (O(V*E^2) (it could be O(E^2) but the linear `Blocks`

check made it higher) and the implementation is complex.

We could just identify loops and sum all back edges. However, this requires a

dominator tree construction which is more complex. The time complexity can be

decreased to almost linear, though.

This patch just performs cycle cancelling iteratively. Add two members

`traversable` and `incoming` to GCOVArc. There are 3 states:

`!traversable`: blocks not on this line or explored blocks`traversable && incoming == nullptr`: unexplored blocks`traversable && incoming != nullptr`: blocks which are being explored (on the stack)

If an arc points to a block being explored, a cycle has been found.

Let E be the number of arcs. Every time a cycle is found, at least one arc is

saturated (`edgeCount` reduced to 0), so there are at most E cycles. Finding one

cycle takes O(E) time, so the overall time complexity is O(E^2). Note that we

always augment through a back edge and never need to augment its reverse edge so

reverse edges in traditional flow networks are not needed.

Reviewed By: xinhaoyuan

Differential Revision: https://reviews.llvm.org/D93073