As other answers have said, spells generally do what they say they do, and not more than that. Anything above and beyond what is written is a houserule—solely up to your GM. We cannot tell you that polymorph effects can be used this way. Those spells do not say they do, so only your GM can state that they do for your game.
On the other hand, disintegrate doesn’t say that it works this way either—all disintegrate does to a creature target is damage, and if that damage is enough to kill the target, destroys the corpse. In fact, nothing in all of Pathfinder’s official, core rules says that it removes limbs. Limb loss is not a part of the game. Nothing can cause it. Anything your GM says has caused it, is also a houserule.
And as a result, we can’t help you, because Pathfinder doesn’t really address this concern that cannot actually happen within its rules. The only reason regenerate exists is as a legacy thing, and maybe a potential plot hook. In the normal course of things, it heals a condition that cannot happen.
When your GM changes the game (by allowing disintegrate to remove a limb), he or she ought to take responsibility for that change—by considering its ramifications and what else needs to change. For example, restoring a lost limb should not be dramatically more difficult than restoring someone to life— that’s ridiculous. So some regenerative effect should be added at lower levels—probably to lesser restoration.
But you will have to ask your GM about that. If your GM refuses to address this issue, and feels that it’s appropriate to cripple PCs and take players effectively out of the game via “clever” houserule, then I suggest you find a new GM.