[Assignment Tracking][mem2reg] Remove overly defensive assert

[Orlando]

Summary

The assert fires if a store to an alloca with no linked dbg.assigns has linked dbg.assigns. This can happen in the wild due to optimisations dropping the alloca's debug info so we shouldn't assert against it.

[jryans]

Makes sense to me, thanks!

llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

void init(AllocaInst *AI) {
     }
   }
 
   /// Update assignment tracking debug info given for the to-be-deleted store
   /// \p ToDelete that stores to this alloca.
   void updateForDeletedStore(StoreInst *ToDelete, DIBuilder &DIB) const {
     // There's nothing to do if the alloca doesn't have any variables using
     // assignment tracking.
-    if (DbgAssigns.empty()) {
-      assert(at::getAssignmentMarkers(ToDelete).empty());
+    if (DbgAssigns.empty())
       return;
-    }
 
     // Just leave dbg.assign intrinsics in place and remember that we've seen
     // one for each variable fragment.
     SmallSet<DebugVariable, 2> VarHasDbgAssignForStore;
     for (DbgAssignIntrinsic *DAI : at::getAssignmentMarkers(ToDelete))
       VarHasDbgAssignForStore.insert(DebugVariable(DAI));
 
     // It's possible for variables using assignment tracking to have no