Avoid `if` call in the `__parallel_merge_submitter_large::run_parallel_merge()` #270

Job	Run time
HOST,bknd=omp,cmplr=clang++,macos-latest,std=c++17,cfg=release	5m 3s
HOST,bknd=tbb,cmplr=icx-cl,windows-latest,std=c++17,cfg=release	16m 56s
clang-format	21s
HOST,bknd=tbb,cmplr=cl,windows-latest,std=c++17,cfg=release	15m 45s
CPU,bknd=dpcpp,cmplr=icx,windows-latest,std=c++17,cfg=release	5m 12s
CPU,bknd=dpcpp,cmplr=icpx,ubuntu-latest,std=с++17,cfg=release	2m 33s
FPGA_EMU,bknd=dpcpp,cmplr=icpx,ubuntu-latest,std=с++17,cfg=release	3m 4s
HOST,bknd=tbb,cmplr=icpx,ubuntu-latest,std=с++17,cfg=release	11m 26s
HOST,bknd=omp,cmplr=icpx,ubuntu-latest,std=с++17,cfg=release	9m 41s
HOST,bknd=omp,cmplr=g++,ubuntu-latest,std=с++17,cfg=release	6m 5s
HOST,bknd=tbb,cmplr=g++,ubuntu-latest,std=с++17,cfg=release	8m 48s
HOST,bknd=tbb,cmplr=g++,ubuntu-latest,std=с++17,cfg=release	8m 59s
HOST,bknd=tbb,cmplr=clang++,ubuntu-latest,std=с++17,cfg=release	10m 49s
HOST,bknd=omp,cmplr=clang++,ubuntu-latest,std=с++17,cfg=release	8m 35s
HOST,bknd=tbb,cmplr=clang++,ubuntu-latest,std=с++17,cfg=release	10m 58s
HOST,bknd=tbb,cmplr=icpx,ubuntu-latest,std=с++17,cfg=release	11m 40s
HOST,bknd=serial,cmplr=g++,ubuntu-latest,std=с++17,cfg=release	4m 50s
CPU,bknd=dpcpp,cmplr=icpx,ubuntu-latest,std=с++20,cfg=release	2m 39s
HOST,bknd=tbb,cmplr=icpx,ubuntu-latest,std=с++20,cfg=release	17m 11s
HOST,bknd=omp,cmplr=icpx,ubuntu-latest,std=с++20,cfg=release	17m 43s
	2h 58m 18s

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