correct_store_buffer: the store buffer is monotonic

Since the store buffer is monotonic when e = e' and state_leb e e' =
true, and is also monotonic when e <> e' and state_leb e e' = true, and
since (e = e') \/ (e <> e'), then the store buffer is monotonic.

The proof invokes the instr_kind comparison lemma to split the two
cases, and applies the monotonicity lemmas on each corresponding case.

Signed-off-by: Alban Gruin <alban.gruin@irit.fr>