target/mips: Simplify Loongson MULTU.G opcode

target/mips: Simplify Loongson MULTU.G opcode

Since MULTU opcodes don't record the most significant bits
of the infinite result, sign-extending the sources make no
difference in the result.

Once we remove the sign extension of source registers, MULT
and MULTU are identical (as are DMULT and DMULTU).

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20241026175349.84523-3-philmd@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>