selftests/bpf: add verifier sign extension bound computation tests.

selftests/bpf: add verifier sign extension bound computation tests.

This commit adds 3 tests to verify a common compiler generated
pattern for sign extension (r1 <<= 32; r1 s>>= 32).
The tests make sure the register bounds are correctly computed both for
positive and negative register values.

Signed-off-by: Cupertino Miranda  <cupertino.miranda@oracle.com>
Signed-off-by: Andrew Pinski  <andrew.pinski@oss.qualcomm.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Cc: David Faust  <david.faust@oracle.com>
Cc: Jose Marchesi  <jose.marchesi@oracle.com>
Cc: Elena Zannoni  <elena.zannoni@oracle.com>
Link: https://lore.kernel.org/r/20251202180220.11128-3-cupertino.miranda@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>