AMDGPU/MC: Allow disassembling without symbol info

[nhaehnle]

Summary

We would like the UMR debugging tool[0] to be able to provide
disassembly for currently live waves based on plain memory
dumps, and we want to leverage the LLVM disassembler for this.

This mostly works, except that UMR clearly can't provide real
symbol info, so it wants to set DisInfo == nullptr.

[0] https://cgit.freedesktop.org/amd/umr/

[artem.tamazov]

LGTM.

llvm/trunk/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp

bool AMDGPUSymbolizer::tryAddingSymbolicOperand(MCInst &Inst,
   using SymbolInfoTy = std::tuple<uint64_t, StringRef, uint8_t>;
   using SectionSymbolsTy = std::vector<SymbolInfoTy>;
 
   if (!IsBranch) {
     return false;
   }
 
   auto *Symbols = static_cast<SectionSymbolsTy *>(DisInfo);
+  if (!Symbols)
+    return false;
+
   auto Result = std::find_if(Symbols->begin(), Symbols->end(),
                              [Value](const SymbolInfoTy& Val) {
                                 return std::get<0>(Val) == static_cast<uint64_t>(Value)
                                     && std::get<2>(Val) == ELF::STT_NOTYPE;
                              });
   if (Result != Symbols->end()) {
     auto *Sym = Ctx.getOrCreateSymbol(std::get<1>(*Result));
     const auto *Add = MCSymbolRefExpr::create(Sym, Ctx);