`defined?(@ivar)` with Ractors should check more #882

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	8m 56s
make (check, i686)	10m 34s
make (check, --disable-yjit)	7m 34s
make (check, --enable-shared --enable-load-relative)	7m 55s
make (check, --with-shared-gc)	7m 27s
make (test-bundler-parallel)	32m 5s
make (test-bundled-gems)	18m 3s
make (check, ubuntu-20.04)	9m 44s
Ubuntu result	1s
	1h 42m 19s

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