When the inline cache is written, the iv table will contain an entry for
the instance variable. If we get an inline cache hit, then we know the
iv table must contain a value for the index written to the inline cache.
If the index in the inline cache is larger than the list on the object,
but *smaller* than the iv index table on the class, then we can just
eagerly allocate the iv list to be the same size as the iv index table.
This avoids duplicate work of checking frozen as well as looking up the
index for the particular instance variable name.
