arm64: insn: fix compare-and-branch encodings

Fix cbz/cbnz having the mask offset by a bit, and add encodings for
tbz/tbnz so that all branch forms are represented.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Zi Shen Lim <zlim.lnx@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>