[ConstraintElimination] Use stable_sort to sort worklist.

If there are multiple constraints in the same block, at the moment the
order they are processed may be different depending on the sort
implementation.

Use stable_sort to ensure consistent ordering.

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -679,7 +679,7 @@ static bool eliminateConstraints(Function &F, DominatorTree &DT) {
   // come before blocks and conditions dominated by them. If a block and a
   // condition have the same numbers, the condition comes before the block, as
   // it holds on entry to the block.
-  sort(S.WorkList, [](const ConstraintOrBlock &A, const ConstraintOrBlock &B) {
+  stable_sort(S.WorkList, [](const ConstraintOrBlock &A, const ConstraintOrBlock &B) {
     return std::tie(A.NumIn, A.IsBlock) < std::tie(B.NumIn, B.IsBlock);
   });