Ensure that combined appears first on acc.loop

Author: Razvan Lupusoru

mlir/include/mlir/Dialect/OpenACC/OpenACCOps.td

@@ -2197,9 +2197,9 @@ def OpenACC_LoopOp : OpenACC_Op<"loop",
 
   let hasCustomAssemblyFormat = 1;
   let assemblyFormat = [{
+    ( `combined` `(` custom<CombinedConstructsLoop>($combined)^ `)` )?
     oilist(
-        `combined` `(` custom<CombinedConstructsLoop>($combined) `)`
-      | `gang` `` custom<GangClause>($gangOperands, type($gangOperands),
+        `gang` `` custom<GangClause>($gangOperands, type($gangOperands),
             $gangOperandsArgType, $gangOperandsDeviceType,
             $gangOperandsSegments, $gang)
       | `worker` `` custom<DeviceTypeOperandsWithKeywordOnly>(