[SelectionDAG] Add instantiated OPC_EmitInteger and OPC_EmitStringInteger (#73241)

These two opcodes are used to be followed by a MVT operand, which is
always one of i8/i16/i32/i64.

We add instantiated `OPC_EmitInteger` and `OPC_EmitStringInteger` with
i8/i16/i32/i64 so that we can reduce one byte.

We reserve `OPC_EmitInteger` and `OPC_EmitStringInteger` in case that
we may need them someday, though I haven't found one usage after this
change.

Overall this reduces the llc binary size with all in-tree targets by
about 200K.