Handle redundant reg-2-reg moves in X86 backend

- Following instruction patterns emit extra redundant register to register moves.
    MoveF2VL
    MoveF2LEG
    MoveVL2F
    MoveLEG2F
    MoveD2VL
    MoveD2LEG
    MoveVL2D
    MoveLEG2D
- Register allocator is capable of constraining the register mask of DEF operand based on the register mask back propagated from USE operand.