Enable ConstPropLint for promoteds

This fixes the issue wherein the lint didn't fire for promoteds
in the case of SHL/SHR operators in non-optimized builds
and all arithmetic operators in optimized builds

Author: gurry

tests/pass/overflow_checks_off.rs

@@ -1,12 +1,17 @@
 //@compile-flags: -C overflow-checks=off
 
 // Check that we correctly implement the intended behavior of these operators
-// when they are not being overflow-checked.
+// when they are not being overflow-checked at runtime.
 
 // FIXME: if we call the functions in `std::ops`, we still get the panics.
 // Miri does not implement the codegen-time hack that backs `#[rustc_inherit_overflow_checks]`.
 // use std::ops::*;
 
+
+// Disable _compile-time_ overflow linting
+// so that we can test runtime overflow checks
+ #![allow(arithmetic_overflow)]
+
 fn main() {
     assert_eq!(-{ -0x80i8 }, -0x80);