Since all active lanes data should be preserved while splitting vector registers
that might contain SGPR spills we introduce new opcode to be used instead of
TargetOpcode::COPY.
Define a new predicated copy opcode (PRED_COPY) to be used in LiveRangeSplitting.
Later instead of lowering it to corresponding MachineInstrction it will be lowered to COPY
Instruction after the necessary WWM code is inserted around it in SISimplifyPredicatedCopyPass.
This will prevent duplication of lowerCopy() code.