bpf: Add the get_reg_width function

Put calculation of the register value width into a dedicated function.
This function will also be used in a following commit.

Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
Link: https://lore.kernel.org/r/20240108205209.838365-8-maxtram95@gmail.com


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