[mlir] AsyncParallelFor: sink constants into the parallel compute function

With complex recursive structure of async dispatch function LLVM can't always propagate constants to the parallel_compute_fn and it often prevents optimizations like loop unrolling and vectorization. We help LLVM by pushing known constants into the parallel_compute_fn explicitly.

Reviewed By: bkramer

Differential Revision: https://reviews.llvm.org/D115263