Fix RAM usage by splitting deeply nested expressions #1273

Job	Run time
build (libgccjit_without_int128.so, master-without-128bit-integers)	10m 25s
build (libgccjit12.so, gcc12, --no-default-features, TEST_FLAGS='-Cpanic=abort -Zpanic-abort-test...	3m 22s
build (libgccjit.so, master)	10m 7s
success_failures	4s
	23m 58s

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