Do not do checking for returning undef value

Upstream does not like to check undef value and clang-format will
fail due to this. Let us remove checking for returning undef value.
A related test is also removed.

Author: Yonghong Song

llvm/lib/Target/BPF/BPFCheckUndefIR.cpp

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// Check 'undef' and 'unreachable' IRs and issue proper warnings.
+// Check 'unreachable' IRs and issue proper warnings.
 //
 //===----------------------------------------------------------------------===//
 
@@ -34,7 +34,6 @@ class BPFCheckUndefIR final : public ModulePass {
 private:
   void BPFCheckUndefIRImpl(Function &F);
   void BPFCheckInst(Function &F, BasicBlock &BB, Instruction &I);
-  void HandleReturnInsn(Function &F, ReturnInst *I);
   void HandleUnreachableInsn(Function &F, BasicBlock &BB, Instruction &I);
 };
 } // End anonymous namespace
@@ -45,17 +44,6 @@ INITIALIZE_PASS(BPFCheckUndefIR, DEBUG_TYPE, "BPF Check Undef IRs", false,
 
 ModulePass *llvm::createBPFCheckUndefIR() { return new BPFCheckUndefIR(); }
 
-void BPFCheckUndefIR::HandleReturnInsn(Function &F, ReturnInst *I) {
-  Value *RetValue = I->getReturnValue();
-  // PoisonValue is a special UndefValue where compiler intentionally to
-  // poisons a value since it shouldn't be used.
-  if (!RetValue || isa<PoisonValue>(RetValue) || !isa<UndefValue>(RetValue))
-    return;
-
-  dbgs() << "WARNING: return undefined value in func " << F.getName()
-         << ", due to uninitialized variable?\n";
-}
-
 void BPFCheckUndefIR::HandleUnreachableInsn(Function &F, BasicBlock &BB,
                                             Instruction &I) {
   // LLVM may create a switch statement with default to a 'unreachable' basic
@@ -87,16 +75,8 @@ void BPFCheckUndefIR::HandleUnreachableInsn(Function &F, BasicBlock &BB,
 
 void BPFCheckUndefIR::BPFCheckInst(Function &F, BasicBlock &BB,
                                    Instruction &I) {
-  switch (I.getOpcode()) {
-  case Instruction::Ret:
-    HandleReturnInsn(F, cast<ReturnInst>(&I));
-    break;
-  case Instruction::Unreachable:
+  if (I.getOpcode() == Instruction::Unreachable)
     HandleUnreachableInsn(F, BB, I);
-    break;
-  default:
-    break;
-  }
 }
 
 void BPFCheckUndefIR::BPFCheckUndefIRImpl(Function &F) {