[RepoHashAlgorithm] A diamond path leading to a loop did not cache what it should have.

[MaggieYingYi]

The new cached hash algorithm is implemented in commit 000ebc2.
The algorithm could get further improvement if it solve the following issue.

Start with the following IR graph:

//       A
//       |
//   +-------+
//   |       |
//   v       v
//   D-----> B <----+
//           |      |
//           v      |
//           C ----->
//

Here we have a loop formed by “b → c → b” which leads directly from a ("a → b") and indirectly via d (“a → d → b”). The current algorithm would consider “d” to be later/deeper than “b” and thus consider it to lie inside the loop and not cached, which is clearly wrong.

[paulhuggett]

Please note that this is a duplicate of this PR.