Fix not applicable on ambiguous ident pat for merge_match_arms

[A4-Tacks]

Example

enum X {
    A,
    B,
    C,
}
use X::*;
fn main() {
    match A {
        $0A => todo!(),
        B => todo!(),
        C => todo!(),
    }
}

Before this PR

Assist not applicable

After this PR

enum X {
    A,
    B,
    C,
}
use X::*;
fn main() {
    match A {
        A | B => todo!(),
        C => todo!(),
    }
}