coverage: Replace the old span refiner with a single function

As more and more of the span refiner's functionality has been pulled out into
separate early passes, it has finally reached the point where we can remove the
rest of the old `SpansRefiner` code, and replace it with a single
modestly-sized function.

tests/coverage/loop-break.coverage

@@ -1,7 +1,7 @@
    LL|       |//@ edition: 2021
    LL|       |
    LL|      1|fn main() {
-   LL|      1|    loop {
+   LL|       |    loop {
    LL|      1|        if core::hint::black_box(true) {
    LL|      1|            break;
    LL|      0|        }