valueForLength() cannot do anything with min-intrinsic.
Length::intrinsic() doesn't return true for min-intrinsic. isLegacyIntrinsic()
does. But use isIntrinsicOrAuto() to catch both intrinsic, legacy-intrinsic and
auto values, since that seems to be a rather popular way of doing things.
Nobody seems to call isLegacyIntrinsic() directly. Regarding auto values here,
it doesn't really matter whether we call valueForLength() or not, since they'll
resolve to 0 anyway.