[SelectionDAG] Add space-optimized forms of OPC_EmitCopyToReg (#73293)

These new opcodes implicitly indicate the RecNo.

The old `OPC_EmitCopyToReg2` is renamed to `OPC_EmitCopyToRegTwoByte`.

Overall this reduces the llc binary size with all in-tree targets by
about 33K (most are from RISCV target).