BenchmarkFastRegexMatcher: report cost metric after ResetTimer

[dimitarvdimitrov]

ReportMetric wasn't showing the cost in benchmark output. Perhaps because it was called before ResetTimer.

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Note

Moves the b.ReportMetric(m.SingleMatchCost(), "cost") call in BenchmarkFastRegexMatcher to after b.ResetTimer() and the loop so the cost metric is emitted.

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