Reland "Use a stable-sort when combining bases"

[saugustine]

The pre-merge failures are all related to riscV, or timeouts, or other unrelated things.

https://reviews.llvm.org/harbormaster/unit/154859/

llvm/lib/Target/ARM/ARMISelLowering.cpp

if (TryCombineBaseUpdate(Target, User, /*SimpleConstIncOnly=*/true, DCI))
       return SDValue();
     ++I;
   }
   BaseUpdates.resize(NumValidUpd);
 
   // Try to fold with other users. Non-constant updates are considered
   // first, and constant updates are sorted to not break a sequence of
   // strided accesses (if there is any).
-  std::sort(BaseUpdates.begin(), BaseUpdates.end(),
-            [](BaseUpdateUser &LHS, BaseUpdateUser &RHS) {
+  std::stable_sort(BaseUpdates.begin(), BaseUpdates.end(),
+                   [](const BaseUpdateUser &LHS, const BaseUpdateUser &RHS) {
                      return LHS.ConstInc < RHS.ConstInc;
                    });
   for (BaseUpdateUser &User : BaseUpdates) {
     if (TryCombineBaseUpdate(Target, User, /*SimpleConstIncOnly=*/false, DCI))
       return SDValue();
   }
   return SDValue();
 }
 
 static SDValue PerformVLDCombine(SDNode *N,