bpf: track immediate values written to stack by BPF_ST instruction

For aligned stack writes using BPF_ST instruction track stored values
in a same way BPF_STX is handled, e.g. make sure that the following
commands produce similar verifier knowledge:

  fp[-8] = 42;             r1 = 42;
                       fp[-8] = r1;

This covers two cases:
 - non-null values written to stack are stored as spill of fake
   registers;
 - null values written to stack are stored as STACK_ZERO marks.

Previously both cases above used STACK_MISC marks instead.

Some verifier test cases relied on the old logic to obtain STACK_MISC
marks for some stack values. These test cases are updated in the same
commit to avoid failures during bisect.

Signed-off-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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