[mlir] Async: Add numWorkerThreads argument to createAsyncParallelForPass

Add an option to pass the number of worker threads to select the number of async regions for parallel for transformation.

std::unique_ptr<OperationPass<FuncOp>> createAsyncParallelForPass(int numWorkerThreads);

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D92835