[mlir][Inliner] Use llvm::parallelForEach instead of llvm::parallelTransformReduce

llvm::parallelTransformReduce does not schedule work on the caller thread, which becomes very costly for
the inliner where a majority of SCCs are small, often ~1 element. The switch to llvm::parallelForEach solves this,
and also aligns the implementation with the PassManager (which realistically should share the same implementation).

This change dropped compile time on an internal benchmark by ~1(25%) second.

Differential Revision: https://reviews.llvm.org/D96086