[enhancement][DSLX] if-expr followed by `as` should give diagnostic about precedence

### What's hard to do? (limit 100 words)

```
$ ~/opt/xlsynth/latest/dslx_interpreter_main /tmp/if_expr.x
/tmp/if_expr.x:1:56-1:58
0001: fn f(x: bool) -> u64 { if x { u32:42 } else { u32:64 } as u64 }
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^ ParseError: Expected '}', got 'keyword:as'
```

We should give a nice diagnostic like rust does, via the rust playground:

```
error: expected expression, found `as`
--> src/lib.rs:2:29
|
2 | if b { 42 } else { 64 } as u64
| ^^ expected expression
|
help: parentheses are required to parse this as an expression
|
2 | (if b { 42 } else { 64 }) as u64
| +
```

-- https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6b48b619a2f263d65f6adf674f126e62

### Current best alternative workaround (limit 100 words)

Break into two statements.

### Your view of the "best case XLS enhancement" (limit 100 words)

We add a similar diagnostic (otherwise it's confusing since we use expressions for the purposes of ternaries).