RISC-V: Handle non-legitimate address in riscv_legitimize_move

RISC-V: Handle non-legitimate address in riscv_legitimize_move

GCC may generate non-legitimate address due to we allow some
load/store with non-legitimate address in pic.md.

gcc/ChangeLog

2017-11-03  Kito Cheng  <kito.cheng@gmail.com>

        * config/riscv/riscv.c (riscv_legitimize_move): Handle
        non-legitimate address.

From-SVN: r254376