[DAG] OptLevelChanger - fix uninitialized variable analyzer warning (PR44471)

[RKSimon]

Summary

Ensure that OptLevelChanger::SavedFastISel is initialized in the constructor.

This should be NFC - as the same 'same opt level' early-out is used in the destructor as well, so SavedFastISel is only actually referenced in the general case.

[dsanders]

LGTM

llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

class OptLevelChanger {
     SelectionDAGISel &IS;
     CodeGenOpt::Level SavedOptLevel;
     bool SavedFastISel;
 
   public:
     OptLevelChanger(SelectionDAGISel &ISel,
                     CodeGenOpt::Level NewOptLevel) : IS(ISel) {
       SavedOptLevel = IS.OptLevel;
+      SavedFastISel = IS.TM.Options.EnableFastISel;
       if (NewOptLevel == SavedOptLevel)
         return;
       IS.OptLevel = NewOptLevel;
       IS.TM.setOptLevel(NewOptLevel);
       LLVM_DEBUG(dbgs() << "\nChanging optimization level for Function "
                         << IS.MF->getFunction().getName() << "\n");
       LLVM_DEBUG(dbgs() << "\tBefore: -O" << SavedOptLevel << " ; After: -O"
                         << NewOptLevel << "\n");
-      SavedFastISel = IS.TM.Options.EnableFastISel;
       if (NewOptLevel == CodeGenOpt::None) {
         IS.TM.setFastISel(IS.TM.getO0WantsFastISel());
         LLVM_DEBUG(
             dbgs() << "\tFastISel is "
                    << (IS.TM.Options.EnableFastISel ? "enabled" : "disabled")
                    << "\n");
       }
     }