[mlir-opt] Delay pass manager creation until after parsing

Currently the pass manager is created before parsing, which requires an
assumption that the top-level operation will be `builtin.module`.
Delaying the creation allows for using the parsed top-level operation as
the PassManager operation instead.

A followup change will allow for parsing top-level operations other than
`builtin.module`.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D133644