target/mips: Fix gen_mxu_s32ldd_s32lddr

There were two bugs here: (1) the required endianness was
not present in the MemOp, and (2) we were not providing a
zero-extended input to the bswap as semantics required.

The best fix is to fold the bswap into the memory operation,
producing the desired result directly.

Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>