don't match on floating-point literal

Author: David Renshaw

tests/run-pass/union.rs

@@ -62,7 +62,7 @@ fn c() {
         unsafe {
             match v {
                 Value { tag: Tag::I, u: U { i: 0 } } => true,
-                Value { tag: Tag::F, u: U { f: 0.0 } } => true,
+                Value { tag: Tag::F, u: U { f } } if f == 0.0 => true,
                 _ => false,
             }
         }