bpf: Let the verifier assign ids on stack fills

The next commit will allow clearing of scalar ids if no other
register/stack slot has that id. This is because if only one register
has a unique id, it can't participate in bounds propagation and is
equivalent to having no id.

But if the id of a stack slot is cleared by clear_singular_ids() in the
next commit, reading that stack slot into a register will not establish
a link because the stack slot's id is cleared.

This can happen in a situation where a register is spilled and later
loses its id due to a multiply operation (for example) and then the
stack slot's id becomes singular and can be cleared.

Make sure that scalar stack slots have an id before we read them into a
register.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260203165102.2302462-2-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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