Fix some `TODO` in `merge` and `merge_sort` algorithms #634

Job	Run time
clang-format	19s
CPU,bknd=dpcpp,cmplr=icpx,ubuntu-latest,std=с++17,cfg=release	15m 3s
FPGA_EMU,bknd=dpcpp,cmplr=icpx,ubuntu-latest,std=с++17,cfg=release	24m 29s
HOST,bknd=tbb,cmplr=icpx,ubuntu-latest,std=с++17,cfg=release	11m 48s
HOST,bknd=omp,cmplr=icpx,ubuntu-latest,std=с++17,cfg=release	9m 35s
HOST,bknd=omp,cmplr=clang++,macos-latest,std=c++17,cfg=release	4m 55s
HOST,bknd=tbb,cmplr=icx-cl,windows-latest,std=c++17,cfg=release	17m 51s
HOST,bknd=omp,cmplr=g++,ubuntu-latest,std=с++17,cfg=release	6m 25s
HOST,bknd=tbb,cmplr=cl,windows-latest,std=c++17,cfg=release	17m 35s
HOST,bknd=tbb,cmplr=g++,ubuntu-latest,std=с++17,cfg=release	9m 33s
CPU,bknd=dpcpp,cmplr=icx,windows-latest,std=c++17,cfg=release	11m 43s
HOST,bknd=tbb,cmplr=g++,ubuntu-latest,std=с++17,cfg=release	9m 23s
HOST,bknd=tbb,cmplr=clang++,ubuntu-latest,std=с++17,cfg=release	11m 25s
HOST,bknd=omp,cmplr=clang++,ubuntu-latest,std=с++17,cfg=release	7m 58s
HOST,bknd=tbb,cmplr=clang++,ubuntu-latest,std=с++17,cfg=release	11m 7s
HOST,bknd=tbb,cmplr=icpx,ubuntu-latest,std=с++17,cfg=release	11m 22s
HOST,bknd=serial,cmplr=g++,ubuntu-latest,std=с++17,cfg=release	5m 15s
CPU,bknd=dpcpp,cmplr=icpx,ubuntu-latest,std=с++20,cfg=release	26m 44s
HOST,bknd=tbb,cmplr=icpx,ubuntu-latest,std=с++20,cfg=release	17m 32s
HOST,bknd=omp,cmplr=icpx,ubuntu-latest,std=с++20,cfg=release	18m 16s
	4h 8m 18s

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