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,

compiler/rustc_middle/src/mir/terminator.rs

@@ -78,6 +78,13 @@ impl SwitchTargets {
     pub fn all_targets_mut(&mut self) -> &mut [BasicBlock] {
         &mut self.targets
     }
+
+    /// Finds the `BasicBlock` to which this `SwitchInt` will branch given the
+    /// specific value.  This cannot fail, as it'll return the `otherwise`
+    /// branch if there's not a specific match for the value.
+    pub fn target_for_value(&self, value: u128) -> BasicBlock {
+        self.iter().find_map(|(v, t)| (v == value).then_some(t)).unwrap_or_else(|| self.otherwise())
+    }
 }
 
 pub struct SwitchTargetsIter<'a> {