Rename `_11` and `_12` suffixes into `_1_1` and `_1_2`

Rename _11 and _12 into _1_1 and _1_2, respectively. These suffixes are used for A → B1 and A → B2 variants of A ↔ B1 ∧ B2 lemmas.

The previous suffixes looked like lemma number 11 and 12, respectively.