Set isRequired true for CFGViewer/CFGPrinter passes

[wenju]

Summary

This PR allows function's cfg to be viewed/printed even if the function
has optnone attribute.

[MaskRay]

Consider a -passes=dot-cfg-quiet test. See llvm/test/Other for some examples.

[wenju]

update llvm/test/Other/cfg-printer-filter.ll to test optnone

llvm/include/llvm/Analysis/CFGPrinter.h

 #include "llvm/Support/DOTGraphTraits.h"
 #include "llvm/Support/FormatVariadic.h"
 
 namespace llvm {
 template <class GraphType> struct GraphTraits;
 class CFGViewerPass : public PassInfoMixin<CFGViewerPass> {
 public:
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+  static bool isRequired() { return true; }
 };
 
 class CFGOnlyViewerPass : public PassInfoMixin<CFGOnlyViewerPass> {
 public:
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+  static bool isRequired() { return true; }
 };
 
 class CFGPrinterPass : public PassInfoMixin<CFGPrinterPass> {
 public:
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+  static bool isRequired() { return true; }
 };
 
 class CFGOnlyPrinterPass : public PassInfoMixin<CFGOnlyPrinterPass> {
 public:
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+  static bool isRequired() { return true; }
 };
 
 class DOTFuncInfo {
 private:
   const Function *F;
   const BlockFrequencyInfo *BFI;
   const BranchProbabilityInfo *BPI;
   uint64_t MaxFreq;