AMDGPU: Fix crash when dumping unknown opcode

[arsenm]

Summary

I'm for some reason having a problem producing a test.
It should be the same as test/MC/X86/invalid_opcode.s,
but llvm-mc seems to ignore random bytes.

[vpykhtin]

LGTM.

[arsenm]

r267225

tools/llvm-objdump/llvm-objdump.cpp

 public:
   void printInst(MCInstPrinter &IP,
                  const MCInst *MI,
                  ArrayRef<uint8_t> Bytes,
                  uint64_t Address,
                  raw_ostream &OS,
                  StringRef Annot,
                  MCSubtargetInfo const &STI) override {
+    if (!MI) {
+      OS << " <unknown>";
+      return;
+    }
+
     SmallString<40> InstStr;
     raw_svector_ostream IS(InstStr);
 
     IP.printInst(MI, IS, "", STI);
 
     OS << left_justify(IS.str(), 60) << format("// %012" PRIX64 ": ", Address);
     typedef support::ulittle32_t U32;
     for (auto D : makeArrayRef(reinterpret_cast<const U32*>(Bytes.data()),