Fix RAM usage by splitting deeply nested expressions #3024

Job	Run time
build_system	24s
spell_check	11s
duplicates	5s
build (gcc-15.deb, --extended-rand-tests)	10m 1s
build (gcc-15.deb, --test-libcore)	18m 3s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	15m 10s
build (gcc-15.deb, --extended-regex-tests)	11m 50s
build (gcc-15.deb, --extended-regex-example-tests)	6m 28s
build (gcc-15.deb, --projects)	11m 25s
build (gcc-15-without-int128.deb, --std-tests)	6m 14s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	14m 2s
build (gcc-15-without-int128.deb, --extended-rand-tests)	10m 0s
build (gcc-15.deb, --std-tests)	7m 35s
build (gcc-15-without-int128.deb, --extended-regex-example-tests)	7m 38s
build (gcc-15-without-int128.deb, --test-libcore)	17m 14s
build (gcc-15-without-int128.deb, --extended-regex-tests)	13m 5s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	13m 45s
build (gcc-15-without-int128.deb, --projects)	12m 21s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	15m 24s
success	2s
	3h 10m 57s

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