[MLIR] Add RecursiveSideEffects to Loops::ParallelOp.

[tpopp]

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.

[herhut]

Thanks!

[tpopp]

Creating a new revision to confirm that build failures were unrelated to my change.

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

OpBuilder getElseBodyBuilder() {
                        results().empty() ? std::prev(body.end()) : body.end());
     }
   }];
 }
 
 def ParallelOp : Loop_Op<"parallel",
     [AttrSizedOperandSegments,
      DeclareOpInterfaceMethods<LoopLikeOpInterface>,
+     RecursiveSideEffects,
      SingleBlockImplicitTerminator<"YieldOp">]> {
   let summary = "parallel for operation";
   let description = [{
     The "loop.parallel" operation represents a loop nest taking 4 groups of SSA
     values as operands that represent the lower bounds, upper bounds, steps and
     initial values, respectively. The operation defines a variadic number of
     SSA values for its induction variables. It has one region capturing the
     loop body. The induction variables are represented as an argument of this