Implement popcount in flattening back-end #11319

Job	Run time
check-ubuntu-24_04-make-clang	26m 7s
check-ubuntu-24_04-cmake-gcc-KNOWNBUG	6m 26s
check-ubuntu-24_04-cmake-gcc-THOROUGH	9m 32s
check-ubuntu-24_04-make-clang-smt-z3	7m 12s
check-ubuntu-24_04-make-gcc	21m 24s
check-macos-13-make-clang	49m 10s
check-ubuntu-22_04-cmake-gcc-32bit	16m 37s
check-ubuntu-24_04-arm-cmake-gcc	12m 56s
check-ubuntu-22_04-cmake-gcc	18m 12s
check-ubuntu-24_04-cmake-gcc	17m 45s
check-ubuntu-24_04-cmake-gcc-14	18m 14s
check-ubuntu-22_04-make-clang	19m 44s
check-docker-image	20m 35s
check-string-table	1m 40s
include-what-you-use	16m 24s
check-macos-14-cmake-clang	1h 49m 41s
check-vs-2025-cmake-build-and-test	39m 19s
check-vs-2022-make-build-and-test	49m 23s
windows-msi-package	9m 20s
	7h 49m 41s

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