[NFC][AVR] Remove unused isMachineVerifierClean()

[xgupta]

Summary

r304320 enabled the machine verifier by default with
EXPENSIVE_CHECKS. Mistakenly AVR was in the one of the targets
excluded from this by overriding TargetMachine::isMachineVerifierClean() to
return false in D33696.

Removing the override as the target passes all lit tests without machine
verifier failure.

[mhjacobson]

Do we happen to know what change(s) made this pass work correctly for AVR?

[xgupta]

IIUC this was added defensively to follow other targets. Seems it was not needed for AVR. And also please commit on my behalf if you are ok with changes, git attribution - "Shivam Gupta <[e-mail]>".

[aykevl]

I actually made several patches to the AVR backend to make it machine verifier clean, so it was certainly not incorrectly disabled. Here are the patches: D96969, D97127, D97172, D97131, D97159. This did in fact find a few actual bugs, so I'm very happy with the work on the machine verifier!

[xgupta]

Oh, Thank you for your efforts @aykevl!

llvm/lib/Target/AVR/AVRTargetMachine.h

public:
   const AVRSubtarget *getSubtargetImpl(const Function &) const override;
 
   TargetLoweringObjectFile *getObjFileLowering() const override {
     return this->TLOF.get();
   }
 
   TargetPassConfig *createPassConfig(PassManagerBase &PM) override;
 
-  bool isMachineVerifierClean() const override {
-    return false;
-  }
-
 private:
   std::unique_ptr<TargetLoweringObjectFile> TLOF;
   AVRSubtarget SubTarget;
 };
 
 } // end namespace llvm
 
 #endif // LLVM_AVR_TARGET_MACHINE_H