fix `defined?(@ivar)` with Ractors #887

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	8m 28s
make (check, i686)	11m 31s
make (check, --disable-yjit)	7m 14s
make (check, --enable-shared --enable-load-relative)	7m 39s
make (check, true, /home/runner/ruby_gc, --with-shared-gc=/home/runner/ruby_gc)	7m 48s
make (test-bundler-parallel)	30m 10s
make (test-bundled-gems)	18m 14s
make (check, ubuntu-20.04)	9m 29s
Ubuntu result	1s
	1h 40m 34s

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