[NPM] Make AddDiscriminators pass required

This is to make sure the pass is not skipped at O0 where optnone is
applied to functions by default.

Author: Yuanfang Chen

llvm/include/llvm/Transforms/Utils/AddDiscriminators.h

@@ -24,6 +24,7 @@ class Function;
 class AddDiscriminatorsPass : public PassInfoMixin<AddDiscriminatorsPass> {
 public:
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+  static bool isRequired() { return true; }
 };
 
 } // end namespace llvm

llvm/test/Transforms/AddDiscriminators/basic.ll

@@ -36,7 +36,7 @@ if.end:                                           ; preds = %if.then, %entry
 ; CHECK:   ret void, !dbg ![[END:[0-9]+]]
 }
 
-attributes #0 = { nounwind uwtable "less-precise-fpmad"="false" "frame-pointer"="all" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #0 = { nounwind uwtable noinline optnone "less-precise-fpmad"="false" "frame-pointer"="all" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
 
 !llvm.dbg.cu = !{!0}
 !llvm.module.flags = !{!7, !8}