Detect match arm guard preceded by | in the parser

[estebank]

Detect and recover the following incorrect code:

enum E {
    A,
    B,
}

fn main() {
    match E::A {
        E::A |
        E::B | // This pipe or operand shouldn't be here
        if true => {}
        _ => {}
    }
}

which currently emits

error: expected identifier, found keyword `if`
  --> src/main.rs:10:9
   |
10 |         if true => {}
   |         ^^ expected identifier, found keyword
help: you can escape reserved keywords to use them as identifiers
   |
10 |         r#if true => {}
   |         ^^^^

error: expected one of `=>`, `@`, `if`, or `|`, found `true`
  --> src/main.rs:10:12
   |
10 |         if true => {}
   |            ^^^^ expected one of `=>`, `@`, `if`, or `|` here

The parser should instead detect it is parsing a match arm guard.