ThreadPool: grow the pool only as needed

On my 96-core cloudtop 'machine', it seems unnecessary to always start
96 threads upfront... particularly as the ThreadPool is created even
with -mlir-disable-threading. Things like the resuling spew in GDB and
the obfuscated output of `(gdb) info threads` are my motivation here,
but it probably also doesn't hurt for at least some efficiency metrics to
avoid creating many threads upfront.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D115019