[MergeICmps] Make sorting strongly stable on the rhs.

[courbet]

Summary

Because the sort order was not strongly stable on the RHS, whether the
chain could merge would depend on the order of the blocks in the Phi.

EXPENSIVE_CHECKS would shuffle the blocks before sorting, resulting in
non-deterministic merging.

[RKSimon]

Thanks guys!

llvm/lib/Transforms/Scalar/MergeICmps.cpp

 #ifdef MERGEICMPS_DOT_ON
   errs() << "BEFORE REORDERING:\n\n";
   dump();
 #endif  // MERGEICMPS_DOT_ON
   // Reorder blocks by LHS. We can do that without changing the
   // semantics because we are only accessing dereferencable memory.
   llvm::sort(Comparisons_,
              [](const BCECmpBlock &LhsBlock, const BCECmpBlock &RhsBlock) {
-               return LhsBlock.Rhs() < RhsBlock.Lhs();
+               return std::tie(LhsBlock.Lhs(), LhsBlock.Rhs()) <
+                      std::tie(RhsBlock.Lhs(), RhsBlock.Rhs());
              });
 #ifdef MERGEICMPS_DOT_ON
   errs() << "AFTER REORDERING:\n\n";
   dump();
 #endif  // MERGEICMPS_DOT_ON
 }
 
 #ifdef MERGEICMPS_DOT_ON