*WIP* error sets - correctly resolve inferred error sets

src/ir_print.cpp

@@ -130,6 +130,8 @@ static const char *ir_bin_op_id_str(IrBinOp op_id) {
             return "++";
         case IrBinOpArrayMult:
             return "**";
+        case IrBinOpMergeErrorSets:
+            return "||";
     }
     zig_unreachable();
 }