MachineMemOperand: Don't crash when printing custom source values

[nhaehnle]

Summary

Just print out a really obvious explanation of what's wrong.

This won't help with actual MIR, but fixes selection DAG debug output.

Fixes: r327580, D42377

[thegameg]

Sorry for breaking this. There is already a patch here: https://reviews.llvm.org/D44871.

[nhaehnle]

That looks more thorough than my quick hack. Dropping this one.

lib/CodeGen/MachineOperand.cpp

case PseudoSourceValue::GlobalValueCallEntry:
           OS, /*PrintType=*/false, MST);
       break;
     case PseudoSourceValue::ExternalSymbolCallEntry:
       OS << "call-entry &";
       printLLVMNameWithoutPrefix(
           OS, cast<ExternalSymbolPseudoSourceValue>(PVal)->getSymbol());
       break;
     case PseudoSourceValue::TargetCustom:
-      llvm_unreachable("TargetCustom pseudo source values are not supported");
+      OS << "TODO(cannot print TargetCustom pseudo source values)";
       break;
     }
   }
   MachineOperand::printOperandOffset(OS, getOffset());
   if (getBaseAlignment() != getSize())
     OS << ", align " << getBaseAlignment();
   auto AAInfo = getAAInfo();
   if (AAInfo.TBAA) {