Optimization/buildhist/hist util

[razdoburdin]

Hi,
this small PR provides performance improvement for BuildHistKernel by helping the compiler to generate faster binary. It is the part of #7192. For performance measurements I booked the c6i.12xlarge (24 cores, hyperthreading is off) instance on on Amazon Web Services and used the benchmarks from here.

Each dataset was executed five times. The average run time is calculated by the formula
average = (run1 + run2 + run3 + run4 + run5 - min - max) / 3. I.e. the fastest and the slowest runs are cuted off, then other three are averaged. The summary of measurements are in the following table:
(Note that speed up value of the mlsr dataset is not a representative, due to runtime speed of this dataset is very unstable).

Dataset name	master run1	master run2	master run3	master run4	master run5	master average	this PR run1	this PR run2	this PR run3	this PR run4	this PR run5	this PR average	speedup
abalone	4.10E-01	4.03E-01	4.11E-01	3.99E-01	4.05E-01	4.06E-01	4.02E-01	4.01E-01	3.94E-01	3.97E-01	3.96E-01	3.98E-01	1.02
airline-ohe	4.09E+01	4.20E+01	4.00E+01	3.99E+01	4.20E+01	4.10E+01	3.37E+01	3.37E+01	3.37E+01	3.27E+01	3.39E+01	3.37E+01	1.22
higgs1m	1.04E+01	1.04E+01	1.04E+01	1.04E+01	1.04E+01	1.04E+01	9.26E+00	9.27E+00	9.27E+00	9.32E+00	9.23E+00	9.27E+00	1.12
letters	1.04E+01	1.04E+01	1.04E+01	1.02E+01	1.02E+01	1.03E+01	1.01E+01	1.00E+01	1.00E+01	1.00E+01	1.00E+01	1.00E+01	1.03
mlsr	9.64E+01	9.50E+01	6.80E+01	6.79E+01	6.83E+01	7.71E+01	5.94E+01	5.31E+01	5.36E+01	5.62E+01	9.19E+01	5.64E+01	1.37
mortgage1Q	1.15E+01	1.15E+01	1.16E+01	1.15E+01	1.15E+01	1.15E+01	1.04E+01	1.04E+01	1.16E+01	1.04E+01	1.04E+01	1.04E+01	1.11
plasticc	4.83E-01	4.86E-01	4.85E-01	4.91E-01	4.83E-01	4.85E-01	4.69E-01	4.72E-01	4.74E-01	4.73E-01	4.77E-01	4.73E-01	1.02
santander	1.29E+02	1.29E+02	1.30E+02	1.29E+02	1.29E+02	1.29E+02	1.02E+02	1.03E+02	1.02E+02	1.03E+02	1.02E+02	1.02E+02	1.26
airline	1.22E+02	1.22E+02	1.21E+02	1.21E+02	1.21E+02	1.21E+02	1.17E+02	1.17E+02	1.17E+02	1.20E+02	1.17E+02	1.17E+02	1.04
bosch	7.57E+00	7.59E+00	7.57E+00	7.59E+00	7.58E+00	7.58E+00	7.23E+00	7.23E+00	7.20E+00	7.18E+00	7.17E+00	7.20E+00	1.05
covtype	5.69E+00	5.77E+00	5.81E+00	5.86E+00	6.69E+00	5.81E+00	4.56E+00	3.79E+00	3.96E+00	4.06E+00	3.26E+00	3.94E+00	1.48
epsion	2.13E+02	2.14E+02	2.12E+02	2.11E+02	2.10E+02	2.12E+02	1.83E+02	1.86E+02	1.84E+02	1.87E+02	1.86E+02	1.85E+02	1.14
fraud	2.79E-01	2.92E-01	2.88E-01	2.79E-01	2.77E-01	2.82E-01	2.58E-01	2.59E-01	2.60E-01	2.58E-01	2.58E-01	2.58E-01	1.09
higgs	1.46E+01	1.45E+01	1.45E+01	1.45E+01	1.45E+01	1.45E+01	1.37E+01	1.38E+01	1.41E+01	1.37E+01	1.37E+01	1.37E+01	1.06
year_prediction_msd	1.45E+00	1.45E+00	1.45E+00	1.45E+00	1.45E+00	1.45E+00	1.26E+00	1.26E+00	1.26E+00	1.27E+00	1.26E+00	1.26E+00	1.15
geomean													1.14

[trivialfis]

Thank you for the optimization! Also, the trick is interesting to learn. ;-)