Fix a covnersion from string to bool issue used in an assert

Problem Was exposed by -Wstring-conversion
    

llvm-svn: 270087

llvm/lib/Target/Hexagon/MCTargetDesc/HexagonInstPrinter.cpp

@@ -210,7 +210,7 @@ void HexagonInstPrinter::printSymbol(MCInst const *MI, unsigned OpNo,
     printOperand(MI, OpNo, O);
   } else {
     printOperand(MI, OpNo, O);
-    assert(!"Unknown symbol operand");
+    llvm_unreachable("Unknown symbol operand");
   }
   O << ')';
 }