Shorten comment

yktrace/src/tir.rs

@@ -406,10 +406,7 @@ impl VarRenamer {
         if &place.local == &Local(0) {
             // Replace the default return variable $0 with the variable in the outer context where
             // the return value will end up after leaving the function. This saves us an
-            // instruction when we compile the trace. More importantly however, this guards us
-            // against a future optimisation in rustc that allows SIR to assign to $0 multiple
-            // times and at the beginning of a block, which could lead to another function
-            // overwriting its value (see https://github.com/rust-lang/rust/pull/72205).
+            // instruction when we compile the trace.
             if let Some(v) = self.returns.last() {
                 v.clone()
             } else {