selftests/bpf: Check liveness merge of masks with different widths

The liveness masks of a function instance are as wide as the deepest
half-slot the instance was seen to access. When the same instance is
analyzed again through another call site, the new pass may have settled
on a different width, and merge_instances() has to widen the original
before combining the two. Add a test where the first pass of a callee
reads through a pointer 248 bytes into the main frame and the second
one through a pointer of unknown offset, which reads the whole frame,
and check that the merged result keeps the whole-frame read. The
precise read stays within the first mask word on 64-bit, so the
whole-frame pass is wider under every stack budget and the merge has
to widen the masks; a deeper read would need two words already, which
is all a 512-byte whole-frame read needs on 64-bit, and the widening
would go untested there.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20260924165740.2146806-12-memxor@gmail.com

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