When this opcode is not available in the backend, tcg middle-end
will expand this as a series of 5 opcodes.  So implementing this
saves bytecode space.

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