Reduce ifdefs, share more code between paths, reduce the number of TCG
ops generated.  Avoid re-computing the size of the operation across
gen_pop_T0 and gen_pop_update.

Add forgotten zero-extension in the TARGET_X86_64, !CODE64, ss32 case.
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NRichard Henderson <rth@twiddle.net>