selftests/bpf: Test 32-bit scalar spill pruning in stacksafe()

selftests/bpf: Test 32-bit scalar spill pruning in stacksafe()

Add a test verifying that stacksafe() correctly handles 32-bit scalar
spills when comparing stack states for equivalence during state pruning.

A 32-bit scalar spill creates slot[0-3] = STACK_INVALID and
slot[4-7] = STACK_SPILL. Without the im=4 check in stacksafe(), the
STACK_SPILL vs STACK_MISC mismatch at byte 4 causes pruning to fail,
forcing the verifier to re-explore a path that is provably safe.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260323022410.75444-2-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>