add overflow_checks intrinsic

Author: pitaj

rustc_public/src/mir/body.rs

@@ -1033,6 +1033,8 @@ pub enum RuntimeChecks {
     UbChecks,
     /// cfg!(contract_checks), but at codegen time
     ContractChecks,
+    /// cfg!(overflow_checks), but at codegen time
+    OverflowChecks,
 }
 
 impl Operand {

rustc_public/src/unstable/convert/stable/mir.rs

@@ -330,6 +330,7 @@ impl<'tcx> Stable<'tcx> for mir::NullOp<'tcx> {
             RuntimeChecks(op) => crate::mir::NullOp::RuntimeChecks(match op {
                 UbChecks => crate::mir::RuntimeChecks::UbChecks,
                 ContractChecks => crate::mir::RuntimeChecks::ContractChecks,
+                OverflowChecks => crate::mir::RuntimeChecks::OverflowChecks,
             }),
         }
     }