Implement JumpThreading pass.

compiler/rustc_middle/src/mir/terminator.rs

@@ -28,6 +28,15 @@ impl SwitchTargets {
         Self { values: smallvec![value], targets: smallvec![then, else_] }
     }
 
+    /// Inverse of `SwitchTargets::static_if`.
+    pub fn as_static_if(&self) -> Option<(u128, BasicBlock, BasicBlock)> {
+        if let &[value] = &self.values[..] && let &[then, else_] = &self.targets[..] {
+            Some((value, then, else_))
+        } else {
+            None
+        }
+    }
+
     /// Returns the fallback target that is jumped to when none of the values match the operand.
     pub fn otherwise(&self) -> BasicBlock {
         *self.targets.last().unwrap()