Avoid creating a ThreadPool in MlirOptMain when `--mlir-disable-threading` option is set

a32300a6 changed it to create a ThreadPool eagerly so that it gets reused
across buffers, however it also made it so that we create a ThreadPool
early even if we're not gonna use it later because of the command line
option `--mlir-disable-threading` is provided.

Fix #53056

Reland 45adf608 after build fixes

Differential Revision: https://reviews.llvm.org/D116848