Make gen_msa() and gen_msa_branch() public declarations
so we can keep calling them once extracted from the big
translate.c in the next commit.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201215225757.764263-18-f4bug@amsat.org>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>