inference: don't allow `SSAValue`s in assignment lhs (#56314)
In `InferenceState` the lhs of a `:=` expression should only contain
`GlobalRef` or `SlotNumber` and no other IR elements. Currently when
`SSAValue` appears in `lhs`, the invalid assignment effect is somehow
ignored, but this is incorrect anyway, so this commit removes that
check. Since `SSAValue` should not appear in `lhs` in the first place,
this is not a significant change though.
