bpf: derive subreg bounds from full bounds when upper 32 bits are constant

Comments in code try to explain the idea behind why this is correct.
Please check the code and comments.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231102033759.2541186-6-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>