CMake

faster merge code for row-major #1114

Job	Run time
full-build (ubuntu-latest, "-DL0_SAMPLING")	30s
full-build (ubuntu-latest, "-DNO_EAGER_DSU")	16s
full-build (ubuntu-latest, "")	31s
full-build (ubuntu-20.04, "-DL0_SAMPLING")	32s
full-build (ubuntu-20.04, "-DNO_EAGER_DSU")	32s
full-build (ubuntu-20.04, "")	31s
	2m 52s

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