Use ALWAYS_INLINE for vm_getinstancevariable #726

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	8m 30s
make (test-bundler-parallel, 50)	23m 54s
make (check, --disable-yjit)	7m 25s
make (check, --enable-shared --enable-load-relative)	7m 58s
make (check, ubuntu-24.04, capi)	9m 22s
make (check, i686)	11m 19s
make (test-bundled-gems)	20m 5s
make-ibm	-1s
Ubuntu result	3s
	1h 28m 35s

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