[naga wgsl-in] Fix parsing break ifs

[Patryk27]

Closes #4982.

Note that #4558 (comment) does not get solved by this commit - in particular, something like this:

fn foo() {
    var a = 0;

    loop {
        let cmp = a < 8;

        if cmp {
            a += 1;
        }

        continuing {
            break if cmp;
        }
    }
}

... still gets miscompiled (when going to glsl, hlsl and msl):

void foo(
) {
    int a_2 = 0;
    bool loop_init_4 = true;
    while(true) {
        if (!loop_init_4) {
            if (a_2 < 8) { // whoopsie - should read `cmp`, not redo the comparison here!
                break;
            }
        }
        loop_init_4 = false;
        int _e2 = a_2;
        bool cmp = _e2 < 8;
        if (cmp) {
            int _e6 = a_2;
            a_2 = _e6 + 1;
        }
    }
    return;
}

... 'cause there's some another bug somewhere, not related to the wgsl frontend, that I can't put my finger on right now (using var cmp instead of let cmp generates correct output, tho).

[teoxoy]

Thanks, looks good!