[MISched][scheduleDump] Use stable_sort to prevent test failures.

When building the compiler with -DLLVM_ENABLE_EXPENSIVE_CHECKS=ON,
sometimes resources that are dumped in scheduled traces gets reordered
even if they are booked in the same cycle. Using `stable_sort`
guarantees that such occasional reordering does not happen.

This change should fix failures like the one seen in
https://lab.llvm.org/buildbot/#/builders/16/builds/49592.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D152800