[LCG] Switch the primary SCC building code to use the negative low-link

values rather than an expensive dense map query to test whether children
have already been popped into an SCC. This matches the incremental SCC
building code. I've also included the assert that I put there but
updated both of their text.

No functionality changed here.

I still don't have any great ideas for sharing the code between the two
implementations, but I may try a brute-force approach to factoring it at
some point.

llvm-svn: 207042