Small mir-opt refactor

Hopefully-non-controversial changes from some not-ready-yet work that I'd figured I'd submit on their own.

compiler/rustc_middle/src/mir/mod.rs

@@ -1504,6 +1504,7 @@ impl Statement<'_> {
     }
 
     /// Changes a statement to a nop and returns the original statement.
+    #[must_use = "If you don't need the statement, use `make_nop` instead"]
     pub fn replace_nop(&mut self) -> Self {
         Statement {
             source_info: self.source_info,