ModGC

Remove dead_stores HashSet and clean up pass #2617

Job	Run time
check (mmtk, release, macos-latest)	10m 59s
check (default, ubuntu-latest)	7m 29s
check (mmtk, release, ubuntu-latest)	10m 56s
check (default, macos-latest)	8m 28s
	37m 52s

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