Compute stats tablesample spends a lot of time in powf()

mmokhtar did perf profiling for COMPUTE STATS TABLESAMPLE and discovered that a lot of time is spent on finalizing HLL intermediates. Most time is spent in powf().

Relevant snippet from AggregateFunctions::HllFinalEstimate() in aggregate-functions-ir.cc:

  for (int i = 0; i < num_buckets; ++i) {
    harmonic_mean += powf(2.0f, -buckets[i]);
    if (buckets[i] == 0) ++num_zero_registers;
  }

Since we're doing a power of 2 using ldexp() should be much more efficient.

I did a microbenchmark and found that ldexp() is >10x faster than powf() for this scenario.