[SelectionDAG] Add space-optimized forms of OPC_EmitRegister (#73291)

The followed byte of `OPC_EmitRegister` is a MVT type, which is
usually i32 or i64.

We add `OPC_EmitRegisterI32` and `OPC_EmitRegisterI64` so that we
can reduce one byte.

Overall this reduces the llc binary size with all in-tree targets by
about 10K.