selftests/bpf: validate precision logic in partial_stack_load_preserves_zeros

selftests/bpf: validate precision logic in partial_stack_load_preserves_zeros

Enhance partial_stack_load_preserves_zeros subtest with detailed
precision propagation log checks. We know expect fp-16 to be spilled,
initially imprecise, zero const register, which is later marked as
precise even when partial stack slot load is performed, even if it's not
a register fill (!).

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231205184248.1502704-10-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>