For the sf version, we are performing two 32-bit bswaps
in either half of the register.  This is equivalent to
performing one 64-bit bswap followed by a rotate.

For the non-sf version, we can remove TCG_BSWAP_IZ
and the preceding zero-extension.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>