target/mips/mxu: Add S32SLT D16SLT D16AVG[R] Q8AVG[R] insns

These instructions are part of pool1, see the grand tree above
in the file. Q8ADD is part of pool1 too but belong to another
category of instructions, thus will be made in later patches.

Signed-off-by: Siarhei Volkau <lis8215@gmail.com>
Message-Id: <20230608104222.1520143-8-lis8215@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>