Introduce MO_LaneMask as new machine operand type. This can be used to
hold liveness infomation at sub-register granularity for register-type
operands. We also introduce a new COPY_LANEMASK instruction that uses
MO_lanemask operand to perform partial copy from source register
opernad.
One such use case of MO_LaneMask can be seen in #151123, where it can be
used to store live regUnits information corresponding to the source
register of the COPY instructions, later can be used during CopyPhysReg
expansion.
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