Fix check_expr_if to point to a more accurate location of the compilation error in some cases

[sgasho]

closes #146190

The problem linked on the issue above lays on how Expr::If is processed.

Currently, Expr::If is processed by depth-first like approach, which is natural considering that Expr::If is a nested structure.

e.g.

fn main() {
    let oa = Some(1);
    let oa2 = Some(1);
    let v = if let Some(a) = oa {
        Some(&a)
    } else if let Some(a) = oa2 {
        &Some(a)
    } else {
        None
    };
    println!("{v:?}");
}

Simplified Expr::If structure for this code is like below.

Expr::If {
  cond: "if let Some(a) = oa",
  then: "Some(&a)",
  else: {
    Expr::If {
      cond: "else if let Some(a) = oa2",
      then: "&Some(a)",
      else: {
        cond: "else",
        then: "None",
        else: None,
      }
    }
  }
}

Current check_expr_if processes this Expr::If by depth-first-search like approach, that is why "if and else have incompatible types" occurs between else if let Some(a) = oa2 and else, not between if let Some(a) = oa and if let Some(a) = oa2.

New approach on this PR is first flatten the Expr::If nested nodes, then process forward.

I created _if.rs and moved the check_expr_if becase this new approach requires a bit more lines of codes.

I added a test case that guarantees the problem on the linked issue has been resolved.
tests/ui/expr/if/if-multi-else-if-incompatible-types-issue-146190.rs

Discussion on Zulip: https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/Struggling.20to.20fix.20issue.20.23146190.20check_expr_if/with/540960074