Tweak implementation of overflow checking assertions

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

Author: tmiasko

src/machine.rs

@@ -822,7 +822,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for MiriMachine<'mir, 'tcx> {
     }
 
     #[inline(always)]
-    fn ignore_checkable_overflow_assertions(ecx: &MiriInterpCx<'mir, 'tcx>) -> bool {
+    fn ignore_optional_overflow_checks(ecx: &MiriInterpCx<'mir, 'tcx>) -> bool {
         !ecx.tcx.sess.overflow_checks()
     }