Improve `opt_merge` performance #404

Job	Run time
pre_job	6s
test-compile (ubuntu-latest, clang-14)	8m 51s
test-compile (ubuntu-latest, gcc-10)	6m 29s
test-compile (ubuntu-latest, clang)	16m 55s
test-compile (ubuntu-latest, gcc)	14m 21s
test-compile (macos-13, clang)	15m 41s
test-compile (ubuntu-22.04, clang-11)	9m 0s
	1h 11m 23s

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