Analyzer produces incomplete set of warnings for conditional control flow with arrays

```c#
void TestMergedArrayElementWithUnknownIndex (int i)
{
Type[] arr = new Type[] { null };
if (i == 1)
arr[0] = GetMethods (); // PublicMethods
else
arr[i] = GetFields (); // UnknownValue
arr[0].RequiresAll (); // Should produce 2 warnings IL2062 and IL2072, analyzer issues only IL2062
}
```
Test comes from [ArrayDataFlow.cs](https://github.com/dotnet/linker/blob/main/test/Mono.Linker.Tests.Cases/DataFlow/ArrayDataFlow.cs)

The multi value at array's index 0 should contain two values after the merge, one coming from the `if` branch, and the other resulting from assigning the value at unknown index in the `else` branch. Thus 2 warnings should be produced.

The analyzer only produces the warning corresponding to assigning the value at unknown index.