bpf: Allow subrange relations for PTR_TO_STACK in regsafe()

regsafe() currently requires an exact match for PTR_TO_STACK registers.
This prevents an explored state from pruning a current state whose
variable-offset range is a subset of the explored range.

This change allows a checkpoint containing a widened stack pointer to
cover narrower offsets reached on subsequent loop iterations,
for example [-56, -8] within [-64, -8].

Merge the PTR_TO_STACK case with PTR_TO_MEM and others, as the set of
necessary checks is almost identical. The only difference is a map_uid
check, which is not needed for PTR_TO_STACK. All locations that
produce PTR_TO_STACK reset map_uid to zero.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261009-scev-minimal-rebase-v3-18-bcba9bc53ac9@gmail.com

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