bpf: detect non null pointer with register operand in JEQ/JNE.

bpf: detect non null pointer with register operand in JEQ/JNE.

This patch adds support to validate a pointer as not null when its
value is compared to a register whose value the verifier knows to be
null.
Initial pattern only verifies against an immediate operand.

Signed-off-by: Cupertino Miranda <cupertino.miranda@oracle.com>
Cc: David Faust  <david.faust@oracle.com>
Cc: Jose Marchesi  <jose.marchesi@oracle.com>
Cc: Elena Zannoni  <elena.zannoni@oracle.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260304195018.181396-3-cupertino.miranda@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>