[InstrProf] Fix BalancedPartitioning when threads are disabled

In https://reviews.llvm.org/D147812 we introduced the class
`BalancedPartitioning` which includes some threading code. The tests in
that diff run forever when built with `-DLLVM_ENABLE_THREADS=OFF` so
some bots were broken.

These tests were skipped in
https://reviews.llvm.org/rGa4845eaf2e9aa18dd900d7cbeff4e5ff52e4b50e
because of this.

This diff disables the threading code if `LLVM_ENABLE_THREADS` is
disabled so we can re-enable the tests.

Reviewed By: luporl

Differential Revision: https://reviews.llvm.org/D152390