Unexpected nullable warning on expression used in tuple assignment

**Version Used**:
17.13.0

**Steps to Reproduce**:

```csharp
#nullable enable

Int32 i;
C? c = null;

if (c is not null)
{
c = c.NoTuple(); // No warning (as expected)
}

if (c is not null)
{
(c, i) = c.Tuple(); // Unexpected CS8602 warning on 'c'
}

class C
{
public C? NoTuple() => throw new NotImplementedException();

public (C?, Int32) Tuple() => throw new NotImplementedException();
}
```

**Diagnostic Id**:
CS8602: Dereference of a possibly null reference.

**Expected Behavior**:
No warnings.

**Actual Behavior**:
A warning on a variable known to be non-null.

**Notes**:
Swapping the order of the tuple elements doesn't produce the warning.

No source file or test is named. Start by running the nullable-enabled reproduction in the issue and tracing the CS8602 diagnostic for the tuple assignment; the fix is complete when the shown assignment produces no warning while the existing behavior remains covered by a regression test.

Written by the indexing model from the issue text.