Prevent miscompilation in trivial loop {}

Ideally, we would want to handle a broader set of cases to fully fix the
underlying bug here. That is currently relatively expensive at compile and
runtime, so we don't do that for now.

compiler/rustc_codegen_ssa/src/traits/intrinsic.rs

@@ -20,7 +20,9 @@ pub trait IntrinsicCallMethods<'tcx>: BackendTypes {
     fn abort(&mut self);
     fn assume(&mut self, val: Self::Value);
     fn expect(&mut self, cond: Self::Value, expected: bool) -> Self::Value;
-    fn sideeffect(&mut self);
+    /// Normally, sideeffect is only emitted if -Zinsert-sideeffect is passed;
+    /// in some cases though we want to emit it regardless.
+    fn sideeffect(&mut self, unconditional: bool);
     /// Trait method used to inject `va_start` on the "spoofed" `VaListImpl` in
     /// Rust defined C-variadic functions.
     fn va_start(&mut self, val: Self::Value) -> Self::Value;