[TableGen] Make OpcodeMappings sort comparator strict weak ordering compliant

This did not satisfy equivalence of transitivity. There was an attempt
to fix it in https://reviews.llvm.org/D58687 but it was not fully
correct. Masks might not be equivalent but be equal according to LessThan lambda

Reviewed By: aeubanks, MaskRay

Differential Revision: https://reviews.llvm.org/D157955

Author: danlark1

llvm/utils/TableGen/CodeGenSchedule.cpp

@@ -369,19 +369,20 @@ processSTIPredicate(STIPredicateFunction &Fn,
                const std::pair<APInt, APInt> &LhsMasks = OpcodeMasks[LhsIdx];
                const std::pair<APInt, APInt> &RhsMasks = OpcodeMasks[RhsIdx];
 
-               auto LessThan = [](const APInt &Lhs, const APInt &Rhs) {
-                 unsigned LhsCountPopulation = Lhs.popcount();
-                 unsigned RhsCountPopulation = Rhs.popcount();
-                 return ((LhsCountPopulation < RhsCountPopulation) ||
-                         ((LhsCountPopulation == RhsCountPopulation) &&
-                          (Lhs.countl_zero() > Rhs.countl_zero())));
+               auto PopulationCountAndLeftBit =
+                   [](const APInt &Other) -> std::pair<int, int> {
+                 return std::pair<int, int>(Other.popcount(),
+                                            -Other.countl_zero());
                };
-
-               if (LhsMasks.first != RhsMasks.first)
-                 return LessThan(LhsMasks.first, RhsMasks.first);
-
-               if (LhsMasks.second != RhsMasks.second)
-                 return LessThan(LhsMasks.second, RhsMasks.second);
+               auto lhsmask_first = PopulationCountAndLeftBit(LhsMasks.first);
+               auto rhsmask_first = PopulationCountAndLeftBit(RhsMasks.first);
+               if (lhsmask_first != rhsmask_first)
+                 return lhsmask_first < rhsmask_first;
+
+               auto lhsmask_second = PopulationCountAndLeftBit(LhsMasks.second);
+               auto rhsmask_second = PopulationCountAndLeftBit(RhsMasks.second);
+               if (lhsmask_second != rhsmask_second)
+                 return lhsmask_second < rhsmask_second;
 
                return LhsIdx < RhsIdx;
              });