bpf: Track may_write flags in liveness

Track stack slots that instructions may modify, in order to account
for such writes when constructing SCEV expressions.

Consume the may_write flag in arg_access_info separately from reads and
definite writes. Include partially covered slots and all candidate
offsets; imprecise writes mark whole candidate frames. Possible writes
do not kill liveness.

Merge analyzed instances as:

  may_write(dst)  |= may_write(src)
  must_write(dst) &= must_write(src)

For each ancestor frame f, summarize callee writes at the callsite:

  may_write(callsite, f) |= OR_{i in callee} may_write(i, f)

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-v3-4-bcba9bc53ac9@gmail.com

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