Slightly refactor constant evaluation and add detection for empty macro expansion and cfged statements.

tests/ui/match_same_arms.rs

@@ -72,6 +72,18 @@ fn main() {
         _ => 1,
     };
 
+    let _ = match 0 {
+        0 => {
+            m!(foo);
+            0
+        },
+        1 => {
+            m!(bar);
+            0
+        },
+        _ => 1,
+    };
+
     let _ = match 0 {
         0 => {
             let mut x = 0;