target/mips: Hard code size with MO_{8|16|32|64}

target/mips: Hard code size with MO_{8|16|32|64}

Temporarily no-op size_memop was introduced to aid the conversion of
memory_region_dispatch_{read|write} operand "unsigned size" into
"MemOp op".

Now size_memop is implemented, again hard coded size but with
MO_{8|16|32|64}. This is more expressive and avoids size_memop calls.

Signed-off-by: Tony Nguyen <tony.nguyen@bt.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-Id: <99c4459d5c1dc9013820be3dbda9798165c15b99.1566466906.git.tony.nguyen@bt.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>