fix(RVCDecoder): add check for zcb reserved space

[TheKiteRunner24]

For zcb arithmetic instruction, x(12, 10) is "111", x(6, 5) is "11" or "10". When x(6, 5) is "11", x(4, 2) = "110" or x(4, 2) = "111" is reserved.

[XiangShanRobot]

[Generated by IPC robot]
commit: 3cbc288

commit	astar	copy_and_run	coremark	gcc	gromacs	lbm	linux	mcf	microbench	milc	namd	povray	wrf	xalancbmk
3cbc288	1.920	0.450	2.701	1.228	2.891	2.473	2.398	0.915	1.429	1.700	3.470	2.729	2.417	3.280

master branch:

commit	astar	copy_and_run	coremark	gcc	gromacs	lbm	linux	mcf	microbench	milc	namd	povray	wrf	xalancbmk
cfa1639		0.450	2.701	1.244		2.473	2.398	0.915	1.429	1.700	3.470	2.729		3.280
0d13456	1.920	0.450	2.701	1.234	2.891	2.473	2.398	0.915	1.429	1.700	3.470	2.729	2.417	3.280
011f1ef	1.917	0.450	2.701	1.241	2.891	2.473	2.398	0.915	1.429	1.700	3.470	2.729	2.417	3.280
96d0318
211986a