NullableWalker violates invariants of slot and slot parents

[gafter]

I tried adding the following assertion

                HashSet<DiagnosticInfo> discardedUseSiteDiagnostics = null;
                Debug.Assert(
                    symbol.Kind != SymbolKind.Field || // we only check fields, but the NullableWalker also works with properties
                    containingSlot == 0 && !symbol.RequiresInstanceReceiver() ||
                    symbol.ContainingType.Equals(variableBySlot[containingSlot].Symbol.GetTypeOrReturnType().Type, TypeCompareKind.AllIgnoreOptions) ||
                    compilation.Conversions.IsBaseClass(variableBySlot[containingSlot].Symbol.GetTypeOrReturnType().Type, symbol.ContainingType, ref discardedUseSiteDiagnostics));

before the following line in LocalDataFlowPass.cs

                variableBySlot[slot] = identifier;

but there are a number of failures (of this assertion) in NullableWalker. It is possible that some of these are symptoms of #20648 but I believe at least the failure in NullableReferenceTypesTests.Conversions_TupleConversions_03 and two other tests is a deeper problem. I am skipping those tests to make progress on #39220, and I removing the assertion for now. The assertion should be readded after #20648 has been fixed and any issues causing the assertion to fail should be reevaluated.