[SCCPSolver] Speed up SCCPSolver by avoiding repeated work list elements

If a value is already the last element of the worklist, then I think that we don't have to add it again, it is not needed to process it repeatedly.

For some long Triton-generated LLVM IR, this can cause a ~100x speedup.

Differential Revision: https://reviews.llvm.org/D153561

llvm/lib/Transforms/Utils/SCCPSolver.cpp

@@ -838,9 +838,13 @@ bool SCCPInstVisitor::markBlockExecutable(BasicBlock *BB) {
 }
 
 void SCCPInstVisitor::pushToWorkList(ValueLatticeElement &IV, Value *V) {
-  if (IV.isOverdefined())
-    return OverdefinedInstWorkList.push_back(V);
-  InstWorkList.push_back(V);
+  if (IV.isOverdefined()) {
+    if (OverdefinedInstWorkList.empty() || OverdefinedInstWorkList.back() != V)
+      OverdefinedInstWorkList.push_back(V);
+    return;
+  }
+  if (InstWorkList.empty() || InstWorkList.back() != V)
+    InstWorkList.push_back(V);
 }
 
 void SCCPInstVisitor::pushToWorkListMsg(ValueLatticeElement &IV, Value *V) {