[GlobalISel] Implement identity transforms for x op x -> x

When we have

```
a = G_OR x, x
```

or

```
b = G_AND y, y
```

We can drop the G_OR/G_AND and just use x/y respectively.

Also update arm64-fallback.ll because there was an or in there which hits this
transformation.

Differential Revision: https://reviews.llvm.org/D77105