Allow macro-expanded macros in trailing expression positions to expand into statements:

```rust
macro_rules! m { () => { let x = 1; x } }
macro_rules! n { () => {
    m!() //< This can now expand into statements
}}
fn main() { n!(); }
```

and revert needless fallout fixes.

src/test/run-pass/simd-intrinsic-generic-cast.rs

@@ -113,7 +113,7 @@ fn main() {
         // product macro
         ($from: ident $(, $from_: ident)*: $($to: ident),*) => {
             fn $from() { unsafe { $( test!($from, $to); )* } }
-            tests!($($from_),*: $($to),*);
+            tests!($($from_),*: $($to),*)
         };
         ($($types: ident),*) => {{
             tests!($($types),* : $($types),*);