In preparation of using the decodetree script, explode
gen_msa_branch() as following:

- OPC_BZ_V              -> BxZ_V(EQ)
- OPC_BNZ_V             -> BxZ_V(NE)
- OPC_BZ_[BHWD]         -> BxZ(false)
- OPC_BNZ_[BHWD]        -> BxZ(true)

Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20201208003702.4088927-10-f4bug@amsat.org>