GVN (MemDep): Pathological or infinite behavior

[alinas]

GVN triggers either a pathological behavior or it's going into an infinite loop in the attached repoducer.

The reason is its use of MemDepAnalysis, specifically 30-50% of the time is spent in getNonLocalPointerDependency, out of which most of the time is in the std::sort called from SortNonLocalDepInfoCache
The cache used keeps expanding to order of tens of thousands entries and I don't see a cap for this or a clear indication this didn't go into an infinite loop.

Reproducer:
bin/opt --passes=gvn LoopInterchange.bc on attached file (note: this is a large input!)

LoopInterchange.bc.txt

[nikic]

#143107 improves this test case.

[nikic]

More precisely, it goes down from 9.3s to 4.6s (release build), so two times faster, but still not exactly fast.