Added GCOVEdge which are simple structs owned by the GCOVFunction that
stores the source and destination GCOVBlocks, as well as the counts.
Changed GCOVBlocks so that it stores a vector of source GCOVEdges and a
vector of destination GCOVEdges, rather than just the block number.
Storing the block number was only useful for knowing the number of edges
and for debug info. Using a struct is useful for traversing the edges,
especially back edges which may be needed later.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196175 91177308-0d34-0410-b5e6-96231b3b80d8