Fix needless_match FP on if-lets

[blyxyas]

Closes #13574

Make sure that needless_match doesn't simplify:

if let Some(_) = a() {
// ..
} else let Some(_) = b() {
// ..
}

to:

a()

changelog: [needless_match]: Fix false-positive on if lets

[rustbot]

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[Centri3]

This'll need to check method calls too, at least. It doesn't really seem like it checks the init expr at all before this, does it? So for example, mixing functions & constants has the same issue.

[blyxyas]

I'm not sure if we should just get two snippets at that point, I'll try doing manual pattern matching, but it might be the simplest solution