[GVNHoist] Fix non-deterministic sort order of PHIs for identical instructions

[mgrang]

Summary

This fixes failure in Transforms/GVNHoist/hoist.ll uncovered by D39245.

lib/Transforms/Scalar/GVNHoist.cpp

void findHoistableCandidates(OutValuesType &CHIBBs, InsKind K,
     // CHIArgs now have the outgoing values, so check for anticipability and
     // accumulate hoistable candidates in HPL.
     for (std::pair<BasicBlock *, SmallVector<CHIArg, 2>> &A : CHIBBs) {
       BasicBlock *BB = A.first;
       SmallVectorImpl<CHIArg> &CHIs = A.second;
       // Vector of PHIs contains PHIs for different instructions.
       // Sort the args according to their VNs, such that identical
       // instructions are together.
-      std::sort(CHIs.begin(), CHIs.end(), cmpVN);
+      std::stable_sort(CHIs.begin(), CHIs.end(), cmpVN);
       auto TI = BB->getTerminator();
       auto B = CHIs.begin();
       // [PreIt, PHIIt) form a range of CHIs which have identical VNs.
       auto PHIIt = std::find_if(CHIs.begin(), CHIs.end(),
                                  [B](CHIArg &A) { return A != *B; });
       auto PrevIt = CHIs.begin();
       while (PrevIt != PHIIt) {
         // Collect values which satisfy safety checks.