Tweak implementation of overflow checking assertions

Extract and reuse logic controlling behaviour of overflow checking
assertions instead of duplicating it three times.

compiler/rustc_middle/src/mir/mod.rs

@@ -1274,6 +1274,13 @@ impl<'tcx> BasicBlockData<'tcx> {
 }
 
 impl<O> AssertKind<O> {
+    /// Returns true if this an overflow checking assertion controlled by -C overflow-checks.
+    pub fn is_optional_overflow_check(&self) -> bool {
+        use AssertKind::*;
+        use BinOp::*;
+        matches!(self, OverflowNeg(..) | Overflow(Add | Sub | Mul | Shl | Shr, ..))
+    }
+
     /// Getting a description does not require `O` to be printable, and does not
     /// require allocation.
     /// The caller is expected to handle `BoundsCheck` separately.
@@ -1998,16 +2005,6 @@ impl BorrowKind {
     }
 }
 
-impl BinOp {
-    /// The checkable operators are those whose overflow checking behavior is controlled by
-    /// -Coverflow-checks option. The remaining operators have either no overflow conditions (e.g.,
-    /// BitAnd, BitOr, BitXor) or are always checked for overflow (e.g., Div, Rem).
-    pub fn is_checkable(self) -> bool {
-        use self::BinOp::*;
-        matches!(self, Add | Sub | Mul | Shl | Shr)
-    }
-}
-
 impl<'tcx> Debug for Rvalue<'tcx> {
     fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
         use self::Rvalue::*;

compiler/rustc_middle/src/mir/syntax.rs

@@ -646,8 +646,7 @@ pub enum TerminatorKind<'tcx> {
     /// When overflow checking is disabled and this is run-time MIR (as opposed to compile-time MIR
     /// that is used for CTFE), the following variants of this terminator behave as `goto target`:
     /// - `OverflowNeg(..)`,
-    /// - `Overflow(op, ..)` if op is a "checkable" operation (add, sub, mul, shl, shr, but NOT
-    /// div or rem).
+    /// - `Overflow(op, ..)` if op is add, sub, mul, shl, shr, but NOT div or rem.
     Assert {
         cond: Operand<'tcx>,
         expected: bool,