[SamplePGO] Support -salvage-stale-profile without probes too (#86116)

Currently -salvage-stale-profile is a no-op if the profile is not
probe-based. We observed that it can help for regular, non-probe- based
profiles too: some of our internal benchmarks show 0.2-0.3% QPS
improvement.

There seems to be no good reason to limit this flag to only work for
probe-based profiles.