feat: add Bitvec.ofInt_ofNat (#5081)

We use `no_index` to work around special-handling of `OfNat.ofNat` in
`DiscrTree`, which has been reported as an issue in
#2867 and is currently in the
process of being fixed in #3684.
As the potential fix seems non-trivial and might need some time to
arrive in-tree, we meanwhile add the `no_index` keyword to the
problematic subterm.

---------

Co-authored-by: Eric Wieser <[email protected]>

src/Init/Data/BitVec/Lemmas.lean

@@ -303,6 +303,9 @@ theorem toInt_ofNat {n : Nat} (x : Nat) :
 @[simp] theorem ofInt_natCast (w n : Nat) :
   BitVec.ofInt w (n : Int) = BitVec.ofNat w n := rfl
 
+@[simp] theorem ofInt_ofNat (w n : Nat) :
+  BitVec.ofInt w (no_index (OfNat.ofNat n)) = BitVec.ofNat w (OfNat.ofNat n) := rfl
+
 theorem toInt_neg_iff {w : Nat} {x : BitVec w} :
     BitVec.toInt x < 0 ↔ 2 ^ w ≤ 2 * x.toNat := by
   simp [toInt_eq_toNat_cond]; omega