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Unshare arrays only when they become shareable #101

Job	Run time
ruby-bench (--zjit-inline-threshold=0 --zjit-call-threshold=2, --warmup=1 --bench=1 --excludes=sh...	15m 5s
make (check, --zjit-disable-hir-opt --zjit-call-threshold=1, -T --zjit-disable-hir-opt -T --zjit-...	13m 30s
make (ruby, combo build test, --enable-yjit --enable-zjit)	5m 40s
make (zjit-check, --enable-yjit=dev --enable-zjit, 1.85.0)	5m 32s
ruby-bench (--zjit-call-threshold=2, --warmup=1 --bench=1 --excludes=shipit, --enable-zjit=dev_no...	12m 55s
make (check, --zjit-call-threshold=1, -T --zjit-call-threshold=1, --enable-zjit=dev)	13m 43s
ZJIT macOS result	2s
	1h 6m 27s