Propagate temporary lifetime extension into if and match.
This PR makes this work:
```rust
let a = if true {
..;
&temp() // used to error, but now gets lifetime extended
} else {
..;
&temp() // used to error, but now gets lifetime extended
};
```
and
```rust
let a = match () {
_ => {
..;
&temp() // used to error, but now gets lifetime extended
}
};
```
to make it consistent with:
```rust
let a = {
..;
&temp() // lifetime is extended
};
```
This is one small part of [the temporary lifetimes work](https://github.com/rust-lang/lang-team/issues/253).
This part is backwards compatible (so doesn't need be edition-gated), because all code affected by this change previously resulted in a hard error.
b3bd7058c1