[mlir][Pass] Fix dropped statistics with nested adaptors.

When running in parallel, nesting more than once caused
statistics to be dropped.

Fix by also preparing "async" pass managers before merging,
as they may also have "async" pass managers within.

Add test checking reported statistics have expected values
with and without threading enabled.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D139459