Classify aliasing constructs in the "store" category

There's no way to guess at compile time if the scalar will be modified, such as in "eval '$_ = 1' for $x->{key}". Practically, there's no way to distinguish the xelem op from the one you get for "$x->{key} = $val". I was going against the interpreter's will and this is my defeat.

This commit partially reverts ddbfd527f4c54458985145aae3a837a8f5868551.