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

[Centri3]

There is eq_expr_value for that purpose, whose body is just:

SpanlessEq::new(cx).deny_side_effects().eq_expr(left, right)

Note that, in this case, any functions will be denied due to potentially having side effects (even if there are none). So, calling it manually might be better. Should work fine I think, even if it's a bit strict?

This is already used here and should be an ok, simple solution (I think).