Currently, many unnecessary samples are populated into MemSamples,
including zero-initialized samples and samples in which the PC address
is not contained in any BinaryFunction. But these samples are totally
skipped during processing and the whole MemSamples vector is cleared
immediately after processing. So, we could just stop populating these
samples into MemSamples, which would reduce maximum resident set size
when processing a large perf.data.
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