[CodeGen][SDAG] Skip preferred extend at O0 (#92643)

This is a pure optimization to avoid redundant extensions, but iterating
over all users is expensive, so don't do this at -O0.

Author: aengelke

llvm/include/llvm/CodeGen/SelectionDAG.h

@@ -469,6 +469,7 @@ class SelectionDAG {
   MachineFunction &getMachineFunction() const { return *MF; }
   const Pass *getPass() const { return SDAGISelPass; }
 
+  CodeGenOptLevel getOptLevel() const { return OptLevel; }
   const DataLayout &getDataLayout() const { return MF->getDataLayout(); }
   const TargetMachine &getTarget() const { return TM; }
   const TargetSubtargetInfo &getSubtarget() const { return MF->getSubtarget(); }

llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp

@@ -222,8 +222,10 @@ void FunctionLoweringInfo::set(const Function &fn, MachineFunction &mf,
         if (!isa<AllocaInst>(I) || !StaticAllocaMap.count(cast<AllocaInst>(&I)))
           InitializeRegForValue(&I);
 
-      // Decide the preferred extend type for a value.
-      PreferredExtendType[&I] = getPreferredExtendForValue(&I);
+      // Decide the preferred extend type for a value. This iterates over all
+      // users and therefore isn't cheap, so don't do this at O0.
+      if (DAG->getOptLevel() != CodeGenOptLevel::None)
+        PreferredExtendType[&I] = getPreferredExtendForValue(&I);
     }
   }