RISC-V: fold redundant code in riscv_ip()

The parsing of 'F' and 'O' .insn operands is pretty redundant. Have only a
single instance each of common code, with the inner switch()es merely
handling the actual value insertion. This in particular simplifies the
addition of new sub-forms.

Reviewed-by: Jiawei <jiawei@iscas.ac.cn>