target/riscv: Merge argument decode for RVC shifti

target/riscv: Merge argument decode for RVC shifti

Special handling for IMM==0 is the only difference between
RVC shifti and RVI shifti.  This can be handled with !function.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>