[llvm] [SROA] Use stable sort for slices to avoid non-determinism (PR #91609)

[via llvm-commits]

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@@ -627,11 +627,8 @@ class AllocaSlices {
   /// everything so that the usual ordering properties of the alloca's slices
   /// hold.
   void insert(ArrayRef<Slice> NewSlices) {
-    int OldSize = Slices.size();
     Slices.append(NewSlices.begin(), NewSlices.end());
-    auto SliceI = Slices.begin() + OldSize;
-    llvm::sort(SliceI, Slices.end());
-    std::inplace_merge(Slices.begin(), SliceI, Slices.end());
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henke9600 wrote:

At first I tried to replace `llvm::sort` with `llvm::stable_sort` but that resulted in a bunch of template errors. Since this function seemed to be extremely cold, I went with this simplification instead. During a full build of clang+lld cross-compiling themselves, this function was only called 3 times in total.

Thinking about this again, I realise that I should probably have replaced `llvm::sort` with `std::stable_sort` (not `llvm::stable_sort`). Is that right?

https://github.com/llvm/llvm-project/pull/91609