[InstrProf] Fix trace reservoir sampling (#152563)
`InstrProfWriter::addTemporalProfileTraces()` did not correctly account
for when the sources traces are sampled, but the reservoir size is
larger than what it was before, meaning there is room for more traces.
Also, if the reservoir size decreased, meaning traces should be
truncated.
Depends on https://github.com/llvm/llvm-project/pull/152550 for the test
refactor
