Auto merge of #77972 - Mark-Simulacrum:side-effect-loop, r=nagisa

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.

Performance results indicate this is not a major regression, if at all, so it should be safe to land.

cc #28728

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;