[MLIR] Add RecursiveSideEffects to Loops::ParallelOp.

Summary:
This is to specify that ParallelOp does not have side effects on its own
but has the effects of all operations executed in its region.

Differential Revision: https://reviews.llvm.org/D78707

mlir/include/mlir/Dialect/LoopOps/LoopOps.td

@@ -259,6 +259,7 @@ def IfOp : Loop_Op<"if",
 def ParallelOp : Loop_Op<"parallel",
     [AttrSizedOperandSegments,
      DeclareOpInterfaceMethods<LoopLikeOpInterface>,
+     RecursiveSideEffects,
      SingleBlockImplicitTerminator<"YieldOp">]> {
   let summary = "parallel for operation";
   let description = [{