Remove undef-sccp.ll and use DiagnosticInfo for warning

The test undef-sccp.ll still uses 'undef' in the IR and clang-format
complains it. In this particular case, 'undef' is generated in sroa
and it needs a lot of other passes to reach sccp. So let us remove
this test. The test itself can be accessed in
  https://gist.github.com/yonghong-song/11be8603ad9422f418f60b41beded047
which can be tested in llvm/test/CodeGen/BPF directory.

DiagnosticInfo() is the recommended interface for warnings comparing to
dbgs().

llvm/lib/Target/BPF/BPFCheckUndefIR.cpp

@@ -12,6 +12,7 @@
 
 #include "BPF.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Type.h"
@@ -69,8 +70,11 @@ void BPFCheckUndefIR::HandleUnreachableInsn(Function &F, BasicBlock &BB,
       return;
   }
 
-  dbgs() << "WARNING: unreachable in func " << F.getName()
-         << ", due to uninitialized variable?\n";
+  F.getContext().diagnose(
+      DiagnosticInfoGeneric(Twine("unreachable in func ")
+                                .concat(F.getName())
+                                .concat(", due to uninitialized variable?"),
+                            DS_Warning));
 }
 
 void BPFCheckUndefIR::BPFCheckInst(Function &F, BasicBlock &BB,