[MachineSink] Fix stable sort comparator (#116705)

Fix the comparator in `stable_sort()` to satisfy the strict weak
ordering requirement.

In #115367 this comparator was
changed to use `getCycleDepth()` when `shouldOptimizeForSize()` is true.
However, I mistakenly changed to logic so that we use `LHSFreq <
RHSFreq` if **either** of them are zero. This causes us to fail the last
requirment (https://en.cppreference.com/w/cpp/named_req/Compare).

> if comp(a, b) == true and comp(b, c) == true then comp(a, c) == true

Author: ellishg

llvm/lib/CodeGen/MachineSink.cpp

@@ -1227,7 +1227,8 @@ MachineSinking::GetAllSortedSuccessors(MachineInstr &MI, MachineBasicBlock *MBB,
       AllSuccs, [&](const MachineBasicBlock *L, const MachineBasicBlock *R) {
         uint64_t LHSFreq = MBFI ? MBFI->getBlockFreq(L).getFrequency() : 0;
         uint64_t RHSFreq = MBFI ? MBFI->getBlockFreq(R).getFrequency() : 0;
-        if (llvm::shouldOptimizeForSize(MBB, PSI, MBFI) || !LHSFreq || !RHSFreq)
+        if (llvm::shouldOptimizeForSize(MBB, PSI, MBFI) ||
+            (!LHSFreq && !RHSFreq))
           return CI->getCycleDepth(L) < CI->getCycleDepth(R);
         return LHSFreq < RHSFreq;
       });