target/riscv: Use DisasExtend in shift operations

target/riscv: Use DisasExtend in shift operations

These operations are greatly simplified by ctx->w, which allows
us to fold gen_shiftw into gen_shift.  Split gen_shifti into
gen_shift_imm_{fn,tl} like we do for gen_arith_imm_{fn,tl}.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210823195529.560295-13-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>