Changing the domain to TCGMemOp makes it easier to interoperate
with other portions of the rest of the translator.

We now only have one domain for size operands inside the translator,
which makes things less confusing all the way around.  There are
still a number of helpers that continue to use the log2-1 domain.
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NRichard Henderson <rth@twiddle.net>