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

Found this while trying to build a LLVM toolchain reproducibly from both
Debian 12 and FreeBSD 14. With these changes they come out bit-by-bit
identical.

Previously there was a mix of stable and unstable sorts for slices, now
only stable sorts are used.

Author: henke9600

llvm/lib/Transforms/Scalar/SROA.cpp

@@ -630,7 +630,7 @@ class AllocaSlices {
     int OldSize = Slices.size();
     Slices.append(NewSlices.begin(), NewSlices.end());
     auto SliceI = Slices.begin() + OldSize;
-    llvm::sort(SliceI, Slices.end());
+    std::stable_sort(SliceI, Slices.end());
     std::inplace_merge(Slices.begin(), SliceI, Slices.end());
   }
 
@@ -5122,7 +5122,7 @@ bool SROA::splitAlloca(AllocaInst &AI, AllocaSlices &AS) {
   }
 
   if (!IsSorted)
-    llvm::sort(AS);
+    llvm::stable_sort(AS);
 
   /// Describes the allocas introduced by rewritePartition in order to migrate
   /// the debug info.