Linker doesn't trim unused locals when the only usage is in `initobj`

Related to https://github.com/mono/linker/issues/1805.

If you have code such as:
```csharp
Unsafe.SkipInit(out Vector128 value);
if (Sse.IsSupported)
{
value = Vector128.Create(value);
}
```

The linker will trim out the initialization but not the `Unsafe.SkipInit(out Vector128 value);` as it doesn't recognize the call today.

If you try to refactor this to something such as:
```csharp
Vector128 value;

if (Sse.IsSupported)
{
value = Vector128.Create(value);
}
else
{
value = default;
}
```

The linker trims it down to just: `Vector128 value = default;` but does not remove the local and therefore does not remove the now unused `Vector128` type even though the only usage of `value` is in an `initobj valuetype [System.Private.CoreLib]System.Runtime.Intrinsics.Vector128`1` instruction.

The linker should likely recognize this scenario and trim the unused local.