target/i386: Improve bswap translation

Use a break instead of an ifdefed else.
There's no need to move the values through s->T0.
Remove TCG_BSWAP_IZ and the preceding zero-extension.

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