[llvm] [BranchFolding] Remove dubious assert from operator< (PR #71639)

[Alexander Monakov via llvm-commits]

================
@@ -485,13 +485,7 @@ BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
     return true;
   if (getBlock()->getNumber() > o.getBlock()->getNumber())
     return false;
-  // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
-  // an object with itself.
-#ifndef _GLIBCXX_DEBUG
-  llvm_unreachable("Predecessor appears twice");
-#else
   return false;
-#endif
----------------
amonakov wrote:

I think the point is to ensure that the array being sorted does not contain a pair of elements that compare equal, in which case it's more efficient to verify that by inspecting the sorted array:
```c++
for (MPIterator i = MergePotentials.begin(), n = std::next(i);
     n != MergePotentials.end();
     i = n, n++)
    assert(*i < *n);
```

https://github.com/llvm/llvm-project/pull/71639