We no longer flatten a block that looks like this:
```rust
match val {
pat => { macro_call!() }
}
```
Currently, rust ignores trailing semicolons in macro expansion in
expression position (see https://github.com/rust-lang/rust/issues/33953)
If this is changed, flattening a block with a macro call may break the
user's code - the trailing semicolon will no longer parse if the macro
call occurs immediately on the right-hand side of the match arm
(e.g. `pat => macro_call!()`)
