tcg: Handle new bswap flags during optimize

Notice when the input is known to be zero-extended and force
the TCG_BSWAP_IZ flag on.  Honor the TCG_BSWAP_OS bit during
constant folding.  Propagate the input to the output mask.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>