tcg/optimize: Fix sign_mask for logical right-shift

tcg/optimize: Fix sign_mask for logical right-shift

The 'sign' computation is attempting to locate the sign bit that has
been repeated, so that we can test if that bit is known zero.  That
computation can be zero if there are no known sign repetitions.

Cc: qemu-stable@nongnu.org
Fixes: 93a967fbb57 ("tcg/optimize: Propagate sign info for shifting")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2248
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>