target/mips: Convert Loongson [D]MULT[U].G opcodes to decodetree

target/mips: Convert Loongson [D]MULT[U].G opcodes to decodetree

Convert the following opcodes to decodetree:

- MULT.G - multiply 32-bit signed integers
- MULTU.G - multiply 32-bit unsigned integers
- DMULT.G - multiply 64-bit signed integers
- DMULTU.G - multiply 64-bit unsigned integers

Now that all opcodes from the extension have been converted, we
can remove completely gen_loongson_integer() and its 2 calls in
decode_opc_special2_legacy() and decode_opc_special3_legacy().

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20241026175349.84523-9-philmd@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>