correct scoping for shadow lints

tests/compile-fail/shadow.rs

@@ -19,4 +19,12 @@ fn main() {
     let x = first(x); //~ERROR: x is shadowed by first(x) which reuses
     let y = 1;
     let x = y; //~ERROR: x is shadowed by y in this declaration
+    
+    let o = Some(1u8);
+    
+    if let Some(p) = o { assert_eq!(1, p); }
+    match o {
+        Some(p) => p, // no error, because the p above is in its own scope
+        None => 0,
+    };
 }