bpf: rename is_branch_taken reg arguments to prepare for the second one

Just taking mundane refactoring bits out into a separate patch. No
functional changes.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Link: https://lore.kernel.org/r/20231102033759.2541186-12-andrii@kernel.org


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