target/riscv: Use pattern groups in insn16.decode

target/riscv: Use pattern groups in insn16.decode

This eliminates about half of the complicated decode
bits within insn_trans/trans_rvc.inc.c.

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