[AMDGPU] Fix a warning

This patch fixes:

  llvm/lib/Target/AMDGPU/AMDGPU.h:297:18: error: private field 'TM' is
  not used [-Werror,-Wunused-private-field]

Author: kazutakahirata

llvm/lib/Target/AMDGPU/AMDGPU.h

@@ -293,11 +293,8 @@ extern char &AMDGPURewriteUndefForPHILegacyPassID;
 
 class AMDGPURewriteUndefForPHIPass
     : public PassInfoMixin<AMDGPURewriteUndefForPHIPass> {
-private:
-  TargetMachine &TM;
-
 public:
-  AMDGPURewriteUndefForPHIPass(TargetMachine &TM) : TM(TM){};
+  AMDGPURewriteUndefForPHIPass() = default;
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
 };
 

llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

@@ -664,7 +664,7 @@ void AMDGPUTargetMachine::registerPassBuilderCallbacks(PassBuilder &PB) {
           return true;
         }
         if (PassName == "amdgpu-rewrite-undef-for-phi") {
-          PM.addPass(AMDGPURewriteUndefForPHIPass(*this));
+          PM.addPass(AMDGPURewriteUndefForPHIPass());
           return true;
         }
         return false;