incr.comp.: Verify stability of incr. comp. hashes and clean up various other things.
The main contribution of this PR is that it adds the `-Z incremental-verify-ich` functionality. Normally, when the red-green tracking system determines that a certain query result has not changed, it does not re-compute the incr. comp. hash (ICH) for that query result because that hash is already known. `-Z incremental-verify-ich` tells the compiler to re-hash the query result and compare the new hash against the cached hash. This is a rather thorough way of
- testing hashing implementation stability,
- finding missing `[input]` annotations on `DepNodes`, and
- finding missing read-edges,
since both a missed read and a missing `[input]` annotation can lead to something being marked as green instead of red and thus will have a different hash than it should have.
Case in point, implementing this verification logic and activating it for all `src/test/incremental` tests has revealed several such oversights, all of which are fixed in this PR.
r? @nikomatsakis
da3fbe750f