Avoid detoast in texteq/textne/byteaeq/byteane for unequal-length strings.

We can get the length of a compressed or out-of-line datum without actually
detoasting it.  If the lengths of two strings are unequal, we can then
conclude they are unequal without detoasting.  That saves considerable work
in an admittedly less-common case, without costing anything much when the
optimization doesn't apply.

Noah Misch