tcg/mips: Support bswap flags in tcg_out_bswap32

tcg/mips: Support bswap flags in tcg_out_bswap32

Merge tcg_out_bswap32 and tcg_out_bswap32s.
Use the flags in the internal uses for loads and stores.

For mips32r2 bswap32 with zero-extension, standardize on
WSBH+ROTR+DEXT.  This is the same number of insns as the
previous DSBH+DSHD+DSRL but fits in better with the flags check.

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