Add support for if statements to suspicious_operation_groupings lint

[Ryan1729]

The suggestions made in #6039 lead to the suspicious_operation_groupings lint which was initially added in #6086, a PR I wrote.

As part of those suggestions, the idea of making the lint trigger on buggy groups of if statements came up. The PR #6086 took a long time, so I figured it would be a good idea to postpone adding the if statement portion, as well as a particular kind of buggy expression. Below are some tests that I wrote to test those features, that would hopefully be useful if/when someone picks this up again.

fn multiple_comparison_types_and_unary_minus(s: &S) -> bool {
    s.a > s.c
    && s.a < -s.c
    && s.b > s.c // `s.c` should be `s.d` here.
    && s.b < -s.d
}

fn across_if_expression(
    s1: &mut S,
    s2: &mut S,
) -> i32 {
    if s1.a < s2.a {
        s1.a
    } else if s1.b < s2.b {
        s1.b
    } else if s1.c < s2.b { // `s2.b` should be `s2.c` here.
        s1.c
    } else if s1.d < s2.d {
        s1.d
    }
}
 
fn across_if_statements_in_an_expr(s1: &mut S, s2: &mut S) -> bool {
    {
        let mut changed = false;
        if s1.a < s2.a {
            s1.a = s2.a;
            changed = true;
        }
        if s1.b < s2.b {
            s1.b = s2.b;
            changed = true;
        }
        if s1.c < s2.b { // `s2.b` should be `s2.c` here.
            s1.c = s2.c;
            changed = true;
        }
        if s1.d < s2.d {
            s1.d = s2.d;
            changed = true;
        }
        changed
    }
}

fn across_if_statements(s1: &mut S, s2: &mut S) {
    if s1.a < s2.a {
        s1.a = s2.a;
    }

    if s1.b < s2.b {
        s1.b = s2.b;
     }

    if s1.c < s2.b { // `s2.b` should be `s2.c` here.
        s1.c = s2.c;
    }

    if s1.d < s2.d {
        s1.d = s2.d;
     }
}
 
fn across_if_expressions_with_a_leading_unary_minus(
    s1: &mut S,
    s2: &mut S,
) -> i32 {
    -(if s1.a < s2.a {
        s1.a
    } else if s1.b < s2.b {
        s1.b
    } else if s1.c < s2.b { // `s2.b` should be `s2.c` here.
        s1.c
    } else if s1.d < s2.d {
        s1.d
    })
}

[giraffate]

@rustbot modify labels: +L-enhancement