coverage: Remove incorrect assertions from counter allocation

These assertions detect situations where a BCB node would have both a physical
counter and one or more in-edge counters/expressions.

For most BCBs that situation would indicate an implementation bug. However,
it's perfectly fine in the case of a BCB having an edge that loops back to
itself.

Given the complexity and risk involved in fixing the assertions, and the fact
that nothing relies on them actually being true, this patch just removes them
instead.

tests/coverage/let_else_loop.coverage

@@ -1,6 +1,6 @@
    LL|       |#![feature(coverage_attribute)]
    LL|       |//@ edition: 2021
-   LL|       |//@ ignore-test
+   LL|       |
    LL|       |// Regression test for <https://github.com/rust-lang/rust/issues/122738>.
    LL|       |// These code patterns should not trigger an ICE when allocating a physical
    LL|       |// counter to a node and also one of its in-edges, because that is allowed