Add parenthesis around suggestions to use `.into()` on cast expr

[estebank]

Given the following code:

fn main() {
    let array = [1, 2, 3];
    test(array.len() as u8);
}

fn test(length: u32) {
    println!("{}", length);
}

rustc suggests

error[E0308]: mismatched types
 --> src/main.rs:3:10
  |
3 |     test(array.len() as u8);
  |          ^^^^^^^^^^^^^^^^^ expected u32, found u8
help: you can cast an `u8` to `u32`, which will zero-extend the source value
  |
3 |     test(array.len() as u8.into());
  |          ^^^^^^^^^^^^^^^^^^^^^^^^

The suggestion should be enclosing the entire expression in parenthesis.

This was introduced in #47247. The solution is to change needs_paren to be inclusive of the precedence of as, in other words, change the < with <= and add a test for the behavior.

[petrochenkov]

Unary prefix operators should be parenthesized as well in general case.
https://play.rust-lang.org/?gist=010eb251f1e518d682fcf7a70ba0a1db&version=undefined

As I said in #47247 AssocOp::As.precedence() needs to be replaced with PREC_POSTFIX.